Fix getrbp()

The following compiler warning is emitted for the BACKTRACE_X86STACK
part of lib/isc/backtrace.c:

    backtrace.c: In function ‘getrbp’:
    backtrace.c:142:1: warning: no return statement in function returning non-void [-Wreturn-type]

While getrbp() stores the value of the RBP register in the RAX register
and thus does attempt to return a value, this is not enough for an
optimizing compiler to always produce the expected result.  With -O2,
the following machine code may be generated in isc_backtrace_gettrace():

    0x00007ffff7b0ff7a <+10>:	mov    %rbp,%rax
    0x00007ffff7b0ff7d <+13>:	mov    $0x17,%eax
    0x00007ffff7b0ff82 <+18>:	retq

The above is equivalent to:

    sp = (void **)getrbp();
    return (ISC_R_NOTFOUND);

and results in the backtrace never getting printed.

Fix by using an intermediate variable.  With this change in place, the
machine code generated with -O2 becomes something like:

    0x00007ffff7af5638 <+24>:	mov    $0x17,%eax
    0x00007ffff7af563d <+29>:	mov    %rbp,%rdx
    0x00007ffff7af5640 <+32>:	test   %rdx,%rdx
    0x00007ffff7af5643 <+35>:	je     0x7ffff7af56bd <isc_backtrace_gettrace+157>
    ...
    0x00007ffff7af56bd <+157>:	retq

(Note that this method of grabbing a stack trace is finicky anyway
because in order for RBP to be relied upon, -fno-omit-stack-frame must
be present among CFLAGS.)
82 jobs for !4325 with 2231-miscellaneous-backtrace-fixes in 37 minutes and 19 seconds (queued for 30 seconds)
detached
Status Job ID Name Coverage
  Precheck
passed #1257693
amd64 linux
autoreconf

00:00:15

passed #1257695
amd64 linux
clang-format

00:00:17

passed #1257696
amd64 linux
coccinelle

00:03:28

passed #1257697
amd64 linux
danger

00:00:13

passed #1257694
amd64 linux
misc

00:00:12

 
  Build
passed #1257711
amd64 linux
clang:asan

00:03:37

passed #1257715
amd64 linux
clang:buster:amd64

00:01:48

passed #1257718
amd64 libvirt
clang:freebsd11:amd64

00:01:44

passed #1257719
amd64 libvirt
clang:freebsd12:amd64

00:01:36

passed #1257720
amd64 libvirt
clang:openbsd:amd64

00:03:52

passed #1257713
amd64 linux
clang:tsan

00:02:23

passed #1257698
amd64 linux
gcc:alpine3.12:amd64

00:01:36

passed #1257710
amd64 linux
gcc:asan

00:03:27

passed #1257703
amd64 linux
gcc:buster:amd64

00:01:33

passed #1257699
amd64 linux
gcc:centos6:amd64

00:01:18

passed #1257700
amd64 linux
gcc:centos7:amd64

00:01:24

passed #1257701
amd64 linux
gcc:centos8:amd64

00:01:29

passed #1257709
amd64 linux
gcc:focal:amd64

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passed #1257714
amd64 linux
gcc:mutexatomics

00:01:31

passed #1257705
amd64 linux
gcc:out-of-tree

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passed #1257704
amd64 linux
gcc:sid:amd64

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passed #1257706
i386 linux
gcc:sid:i386

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passed #1257716
amd64 linux
gcc:softhsm2.4

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passed #1257717
amd64 linux
gcc:softhsm2.6

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passed #1257702
amd64 linux
gcc:stretch:amd64

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passed #1257712
amd64 linux
gcc:tsan

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passed #1257707
amd64 linux
gcc:tumbleweed:amd64

00:01:30

passed #1257708
amd64 linux
gcc:xenial:amd64

00:01:39

passed #1257722
amd64 windows
msvc:windows:amd64

00:03:08

passed #1257721
amd64 linux
nolibtool:sid:amd64

00:02:02

 
  Unit
passed #1257735
amd64 linux
unit:clang:asan

00:01:00

passed #1257739
amd64 linux
unit:clang:buster:amd64

00:00:38

passed #1257742
amd64 libvirt
unit:clang:freebsd11:amd64

00:01:01

passed #1257743
amd64 libvirt
unit:clang:freebsd12:amd64

00:00:59

passed #1257737
amd64 linux
unit:clang:tsan

00:01:33

passed #1257723
amd64 linux
unit:gcc:alpine3.12:amd64

00:00:39

passed #1257734
amd64 linux
unit:gcc:asan

00:00:50

passed #1257728
amd64 linux
unit:gcc:buster:amd64

00:00:55

passed #1257724
amd64 linux
unit:gcc:centos6:amd64

00:00:37

passed #1257725
amd64 linux
unit:gcc:centos7:amd64

00:00:36

passed #1257726
amd64 linux
unit:gcc:centos8:amd64

00:00:38

passed #1257733
amd64 linux
unit:gcc:focal:amd64

00:00:36

passed #1257738
amd64 linux
unit:gcc:mutexatomics

00:00:36

passed #1257729
amd64 linux
unit:gcc:sid:amd64

00:00:36

passed #1257730
i386 linux
unit:gcc:sid:i386

00:00:44

passed #1257740
amd64 linux
unit:gcc:softhsm2.4

00:00:34

passed #1257741
amd64 linux
unit:gcc:softhsm2.6

00:00:37

passed #1257727
amd64 linux
unit:gcc:stretch:amd64

00:00:35

passed #1257736
amd64 linux
unit:gcc:tsan

00:01:47

passed #1257731
amd64 linux
unit:gcc:tumbleweed:amd64

00:00:34

passed #1257732
amd64 linux
unit:gcc:xenial:amd64

00:00:37

passed #1257744
amd64 linux
unit:nolibtool:sid:amd64

00:00:49

 
  System
passed #1257757
amd64 linux
system:clang:asan

00:12:45

passed #1257761
amd64 linux
system:clang:buster:amd64

00:12:11

passed #1257764
amd64 libvirt
system:clang:freebsd11:amd64

00:10:29

passed #1258050
libvirt amd64
system:clang:freebsd12:amd64

00:10:11

passed #1257759
amd64 linux
system:clang:tsan

00:14:02

passed #1258040
linux amd64
system:gcc:alpine3.12:amd64

00:11:56

passed #1257756
amd64 linux
system:gcc:asan

00:13:22

passed #1257750
amd64 linux
system:gcc:buster:amd64

00:12:13

passed #1257746
amd64 linux
system:gcc:centos6:amd64

00:11:38

passed #1257747
amd64 linux
system:gcc:centos7:amd64

00:11:19

passed #1257748
amd64 linux
system:gcc:centos8:amd64

00:12:04

passed #1257755
amd64 linux
system:gcc:focal:amd64

00:11:17

passed #1257760
amd64 linux
system:gcc:mutexatomics

00:11:27

passed #1257751
amd64 linux
system:gcc:sid:amd64

00:11:02

passed #1257752
i386 linux
system:gcc:sid:i386

00:10:12

passed #1257762
amd64 linux
system:gcc:softhsm2.4

00:11:14

passed #1257763
amd64 linux
system:gcc:softhsm2.6

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passed #1257749
amd64 linux
system:gcc:stretch:amd64

00:11:02

passed #1257758
amd64 linux
system:gcc:tsan

00:14:42

passed #1257753
amd64 linux
system:gcc:tumbleweed:amd64

00:10:50

passed #1257754
amd64 linux
system:gcc:xenial:amd64

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passed #1257766
amd64 linux
system:nolibtool:sid:amd64

00:11:38

failed #1257765
amd64 libvirt
system:clang:freebsd12:amd64

00:10:24

failed #1257745
amd64 linux
system:gcc:alpine3.12:amd64

00:11:53

 
  Docs
passed #1257767
amd64 linux
docs

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  Postcheck
passed #1257771
amd64 linux
cppcheck

00:00:21

passed #1257768
amd64 linux
flake8

00:00:20

passed #1257772
amd64 linux
gcov

00:00:33

76.0%
passed #1257769
amd64 linux
pylint

00:00:22

passed #1257770
amd64 linux
scan-build

00:09:05