arm: libbacktrace: Check if the compiler supports __sync atomics

Older versions of the Arm architecture lack support for __sync
operations directly in hardware and require calls into appropriate
operating-system hooks.  But such hooks obviously don't exist in a
freestanding environment.

Consquently, it is incorrect to assume during configure that such
functions will exist and we need a configure-time check to determine
whether or not these routines will work.

libbacktrace:

	* configure.ac: Always check if the compiler supports __sync
	operations.
	* configure: Regenerated.

configure.ac

@@ -189,6 +189,16 @@ AC_CACHE_CHECK([__sync extensions],
 [if test -n "${with_target_subdir}"; then
    case "${host}" in
    hppa*-*-hpux*) libbacktrace_cv_sys_sync=no ;;
+   arm*-*-eabi*)
+     # Older versions of the Arm architecture lack the necessary instructions
+     # for these constructs, so check whether we can use them.
+     AC_LINK_IFELSE(
+       [AC_LANG_PROGRAM([int i;],
+                        [__sync_bool_compare_and_swap (&i, i, i);
+                         __sync_lock_test_and_set (&i, 1);
+                         __sync_lock_release (&i);])],
+       [libbacktrace_cv_sys_sync=yes],
+       [libbacktrace_cv_sys_sync=no]);;
    *) libbacktrace_cv_sys_sync=yes ;;
    esac
  else