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.)
80 jobs for !4325 with 2231-miscellaneous-backtrace-fixes in 1 minute and 23 seconds (queued for 7 seconds)
detached
Status Job ID Name Coverage
  Precheck
passed #1261888
amd64 linux
autoreconf

00:00:15

passed #1261890
amd64 linux
clang-format

00:00:16

canceled #1261891
amd64 linux
coccinelle

00:01:21

passed #1261892
amd64 linux
danger

00:00:16

passed #1261889
amd64 linux
misc

00:00:13

 
  Build
canceled #1261906
amd64 linux
clang:asan

00:00:58

canceled #1261910
amd64 linux
clang:buster:amd64

00:01:03

canceled #1261913
amd64 libvirt
clang:freebsd11:amd64

00:00:53

canceled #1261914
amd64 libvirt
clang:freebsd12:amd64

00:00:52

canceled #1261915
amd64 libvirt
clang:openbsd:amd64

00:00:52

canceled #1261908
amd64 linux
clang:tsan

00:01:03

canceled #1261893
amd64 linux
gcc:alpine3.12:amd64

00:01:04

canceled #1261905
amd64 linux
gcc:asan

00:00:59

canceled #1261898
amd64 linux
gcc:buster:amd64

00:01:02

canceled #1261894
amd64 linux
gcc:centos6:amd64

00:01:03

canceled #1261895
amd64 linux
gcc:centos7:amd64

00:01:03

canceled #1261896
amd64 linux
gcc:centos8:amd64

00:01:03

canceled #1261904
amd64 linux
gcc:focal:amd64

00:01:00

canceled #1261909
amd64 linux
gcc:mutexatomics

00:01:03

canceled #1261900
amd64 linux
gcc:out-of-tree

00:01:01

canceled #1261899
amd64 linux
gcc:sid:amd64

00:01:02

canceled #1261901
i386 linux
gcc:sid:i386

00:01:01

canceled #1261911
amd64 linux
gcc:softhsm2.4

00:01:02

canceled #1261912
amd64 linux
gcc:softhsm2.6

00:00:56

canceled #1261897
amd64 linux
gcc:stretch:amd64

00:01:03

canceled #1261907
amd64 linux
gcc:tsan

00:00:57

canceled #1261902
amd64 linux
gcc:tumbleweed:amd64

00:01:00

canceled #1261903
amd64 linux
gcc:xenial:amd64

00:01:00

canceled #1261917
amd64 windows
msvc:windows:amd64

00:01:19

canceled #1261916
amd64 linux
nolibtool:sid:amd64

00:00:55

 
  Unit
canceled #1261930
amd64 linux
unit:clang:asan

canceled #1261934
amd64 linux
unit:clang:buster:amd64

canceled #1261937
amd64 libvirt
unit:clang:freebsd11:amd64

canceled #1261938
amd64 libvirt
unit:clang:freebsd12:amd64

canceled #1261932
amd64 linux
unit:clang:tsan

canceled #1261918
amd64 linux
unit:gcc:alpine3.12:amd64

canceled #1261929
amd64 linux
unit:gcc:asan

canceled #1261923
amd64 linux
unit:gcc:buster:amd64

canceled #1261919
amd64 linux
unit:gcc:centos6:amd64

canceled #1261920
amd64 linux
unit:gcc:centos7:amd64

canceled #1261921
amd64 linux
unit:gcc:centos8:amd64

canceled #1261928
amd64 linux
unit:gcc:focal:amd64

canceled #1261933
amd64 linux
unit:gcc:mutexatomics

canceled #1261924
amd64 linux
unit:gcc:sid:amd64

canceled #1261925
i386 linux
unit:gcc:sid:i386

canceled #1261935
amd64 linux
unit:gcc:softhsm2.4

canceled #1261936
amd64 linux
unit:gcc:softhsm2.6

canceled #1261922
amd64 linux
unit:gcc:stretch:amd64

canceled #1261931
amd64 linux
unit:gcc:tsan

canceled #1261926
amd64 linux
unit:gcc:tumbleweed:amd64

canceled #1261927
amd64 linux
unit:gcc:xenial:amd64

canceled #1261939
amd64 linux
unit:nolibtool:sid:amd64

 
  System
canceled #1261952
amd64 linux
system:clang:asan

canceled #1261956
amd64 linux
system:clang:buster:amd64

canceled #1261959
amd64 libvirt
system:clang:freebsd11:amd64

canceled #1261960
amd64 libvirt
system:clang:freebsd12:amd64

canceled #1261954
amd64 linux allowed to fail
system:clang:tsan

canceled #1261940
amd64 linux
system:gcc:alpine3.12:amd64

canceled #1261951
amd64 linux
system:gcc:asan

canceled #1261945
amd64 linux
system:gcc:buster:amd64

canceled #1261941
amd64 linux
system:gcc:centos6:amd64

canceled #1261942
amd64 linux
system:gcc:centos7:amd64

canceled #1261943
amd64 linux
system:gcc:centos8:amd64

canceled #1261950
amd64 linux
system:gcc:focal:amd64

canceled #1261955
amd64 linux
system:gcc:mutexatomics

canceled #1261946
amd64 linux
system:gcc:sid:amd64

canceled #1261947
i386 linux
system:gcc:sid:i386

canceled #1261957
amd64 linux
system:gcc:softhsm2.4

canceled #1261958
amd64 linux
system:gcc:softhsm2.6

canceled #1261944
amd64 linux
system:gcc:stretch:amd64

canceled #1261953
amd64 linux allowed to fail
system:gcc:tsan

canceled #1261948
amd64 linux
system:gcc:tumbleweed:amd64

canceled #1261949
amd64 linux
system:gcc:xenial:amd64

canceled #1261961
amd64 linux
system:nolibtool:sid:amd64

 
  Docs
canceled #1261962
amd64 linux
docs

00:00:55

 
  Postcheck
passed #1261966
amd64 linux
cppcheck

00:00:21

passed #1261963
amd64 linux
flake8

00:00:21

canceled #1261967
amd64 linux
gcov

passed #1261964
amd64 linux
pylint

00:00:23

canceled #1261965
amd64 linux
scan-build

00:00:53