configure.ac: autodetect 'pause' instruction presence on sparc

The change fixes the following build failure on sparc T3 and older CPUs:

```
sparc-unknown-linux-gnu-gcc ... -O2 -mcpu=niagara2 ... -c rwlock.c
{standard input}: Assembler messages:
{standard input}:398: Error: Architecture mismatch on "pause ".
{standard input}:398: (Requires v9e|v9v|v9m|m8; requested architecture is v9b.)
make[1]: *** [Makefile:280: rwlock.o] Error 1
```

`pause` insutruction exists only on `-mcpu=niagara4` (`T4`) and upper.

The change adds `pause` configure-time autodetection and uses it if available.
config.h.in got new `HAVE_SPARC_PAUSE` knob. Fallback is a fall-through no-op.

Build-tested on:

- sparc-unknown-linux-gnu-gcc (no `pause`, build succeeds)
- sparc-unknown-linux-gnu-gcc -mcpu=niagara4 (`pause`, build succeeds)

Reported-by: Rolf Eike Beer
Bug: https://bugs.gentoo.org/691708
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
53 jobs for !2243 with sparc-pause in 16 minutes and 12 seconds (queued for 5 seconds)
detached
Status Job ID Name Coverage
  Precheck
passed #299702
amd64 linux
🐞:sid:amd64

00:01:27

passed #299700
amd64 linux
autoreconf:sid:amd64

00:00:35

passed #299701
amd64 linux
misc:sid:amd64

00:00:18

 
  Build
passed #299715
amd64 linux
asan:sid:amd64

00:05:31

passed #299718
amd64 linux
clang:stretch:amd64

00:03:56

passed #299719
i386 linux
clang:stretch:i386

00:02:20

passed #299703
amd64 linux
docs:sid:amd64

00:01:20

passed #299704
amd64 linux
gcc:alpine3.10:amd64

00:03:43

passed #299714
amd64 linux
gcc:bionic:amd64

00:02:08

passed #299709
i386 linux
gcc:buster:amd64

00:03:42

passed #299705
amd64 linux
gcc:centos6:amd64

00:03:16

passed #299706
amd64 linux
gcc:centos7:amd64

00:03:27

passed #299712
amd64 linux
gcc:fedora30:amd64

00:03:21

passed #299707
amd64 linux
gcc:jessie:amd64

00:03:20

passed #299710
amd64 linux
gcc:sid:amd64

00:02:29

passed #299711
i386 linux
gcc:sid:i386

00:02:42

passed #299708
amd64 linux
gcc:stretch:amd64

00:02:19

passed #299713
amd64 linux
gcc:xenial:amd64

00:03:17

passed #299717
amd64 linux
mutexatomics:sid:amd64

00:03:22

passed #299721
amd64 linux
nolibtool:sid:amd64

00:02:37

passed #299720
amd64 linux
pkcs11:sid:amd64

00:02:08

passed #299716
amd64 linux
rwlock:sid:amd64

00:01:31

 
  Test
passed #299744
amd64 linux
system:asan:sid:amd64

00:09:10

passed #299722
amd64 linux
system:gcc:alpine3.10:amd64

00:07:25

passed #299742
amd64 linux
system:gcc:bionic:amd64

00:07:02

passed #299732
i386 linux
system:gcc:buster:amd64

00:07:48

passed #299724
amd64 linux
system:gcc:centos6:amd64

00:06:47

passed #299726
amd64 linux
system:gcc:centos7:amd64

00:07:06

passed #299738
amd64 linux
system:gcc:fedora30:amd64

00:07:57

passed #299728
amd64 linux
system:gcc:jessie:amd64

00:06:33

passed #299734
amd64 linux
system:gcc:sid:amd64

00:07:45

passed #299736
i386 linux
system:gcc:sid:i386

00:07:58

passed #299730
amd64 linux
system:gcc:stretch:amd64

00:06:40

passed #299740
amd64 linux
system:gcc:xenial:amd64

00:06:47

passed #299751
amd64 linux
system:nolibtool:sid:amd64

00:08:32

passed #299749
amd64 linux
system:pkcs11:sid:amd64

00:07:04

passed #299746
amd64 linux
system:rwlock:sid:amd64

00:07:08

passed #299745
amd64 linux
unit:asan:sid:amd64

00:01:16

passed #299748
amd64 linux
unit:clang:stretch:amd64

00:00:45

passed #299723
amd64 linux
unit:gcc:alpine3.10:amd64

00:01:06

passed #299743
amd64 linux
unit:gcc:bionic:amd64

00:00:58

passed #299733
i386 linux
unit:gcc:buster:amd64

00:00:44

passed #299725
amd64 linux
unit:gcc:centos6:amd64

00:00:55

passed #299727
amd64 linux
unit:gcc:centos7:amd64

00:01:00

passed #299739
amd64 linux
unit:gcc:fedora30:amd64

00:01:10

passed #299729
amd64 linux
unit:gcc:jessie:amd64

00:00:30

passed #299735
amd64 linux
unit:gcc:sid:amd64

00:01:00

passed #299737
i386 linux
unit:gcc:sid:i386

00:01:07

passed #299731
amd64 linux
unit:gcc:stretch:amd64

00:00:47

passed #299741
amd64 linux
unit:gcc:xenial:amd64

00:01:00

passed #299752
amd64 linux
unit:nolibtool:sid:amd64

00:01:05

passed #299750
amd64 linux
unit:pkcs11:sid:amd64

00:00:58

passed #299747
amd64 linux
unit:rwlock:sid:amd64

00:01:00