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>

config.h.in

@@ -357,6 +357,9 @@
 /* Define to 1 if you have the `setresuid' function. */
 #undef HAVE_SETRESUID
 
+/* define if the SPARC pause instruction is available */
+#undef HAVE_SPARC_PAUSE
+
 /* define if struct stat has st_mtim.tv_nsec field */
 #undef HAVE_STAT_NSEC
 

configure

@@ -13872,6 +13872,39 @@ rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext ;; #(
      ;;
 esac
 
+#
+# Check for pause support on SPARC processors
+#
+case $host in #(
+  sparc*) :
+    { $as_echo "$as_me:${as_lineno-$LINENO}: checking for pause instruction support" >&5
+$as_echo_n "checking for pause instruction support... " >&6; }
+	 cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+__asm__ __volatile__ ("pause")
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }
+
+$as_echo "#define HAVE_SPARC_PAUSE 1" >>confdefs.h
+
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext ;; #(
+  *) :
+     ;;
+esac
+
 for ac_func in sysctlbyname
 do :
   ac_fn_c_check_func "$LINENO" "sysctlbyname" "ac_cv_func_sysctlbyname"

configure.ac

@@ -415,6 +415,20 @@ AS_CASE([$host],
 			[define if the ARM yield instruction is available])],
 	     [AC_MSG_RESULT([no])])])
 
+#
+# Check for pause support on SPARC processors
+#
+AS_CASE([$host],
+	[sparc*],
+	[AC_MSG_CHECKING([for pause instruction support])
+	 AC_COMPILE_IFELSE(
+	     [AC_LANG_PROGRAM([[]],
+			     [[__asm__ __volatile__ ("pause")]])],
+	     [AC_MSG_RESULT([yes])
+	      AC_DEFINE([HAVE_SPARC_PAUSE], [1],
+			[define if the SPARC pause instruction is available])],
+	     [AC_MSG_RESULT([no])])])
+
 AC_CHECK_FUNCS([sysctlbyname])
 
 #

lib/isc/rwlock.c

@@ -152,7 +152,7 @@ isc_rwlock_destroy(isc_rwlock_t *rwl) {
 # define isc_rwlock_pause() __asm__ __volatile__ ("yield")
 #elif defined(sun) && (defined(__sparc) || defined(__sparc__))
 # define isc_rwlock_pause() smt_pause()
-#elif defined(__sparc) || defined(__sparc__)
+#elif (defined(__sparc) || defined(__sparc__)) && HAVE_SPARC_PAUSE
 # define isc_rwlock_pause() __asm__ __volatile__ ("pause")
 #elif defined(__ppc__) || defined(_ARCH_PPC)  ||			\
 	defined(_ARCH_PWR) || defined(_ARCH_PWR2) || defined(_POWER)