[Patch AArch64] Add __ARM_FEATURE_ATOMICS

This keeps coming up repeatedly and the ACLE has finally added
__ARM_FEATURE_ATOMICS for the LSE feature in GCC. This is now part of
the latest ACLE release
(https://developer.arm.com/docs/101028/latest/5-feature-test-macros)

I know it's late for GCC-9 but this is a simple macro which need not
wait  for another year.

Ok for trunk and to backport to all release branches ?

Tested with a simple build and a smoke test.

PR target/86538
* config/aarch64/aarch64-c.c (aarch64_update_cpp_builtins): Define
__ARM_FEATURE_ATOMICS

From-SVN: r270686

gcc/ChangeLog

+2019-04-30  Ramana Radhakrishnan  <ramana.radhakrishnan@arm.com>
+
+	PR target/86538
+	* config/aarch64/aarch64-c.c (aarch64_update_cpp_builtins):
+	Define __ARM_FEATURE_ATOMICS
+
 2019-04-30  Martin Liska  <mliska@suse.cz>
 
 	* gimple-fold.c (gimple_fold_builtin_memory_op): Change endp

gcc/config/aarch64/aarch64-c.c

@@ -147,6 +147,7 @@ aarch64_update_cpp_builtins (cpp_reader *pfile)
       builtin_define_with_int_value ("__ARM_FEATURE_SVE_BITS", bits);
     }
 
+  aarch64_def_or_undef (TARGET_LSE, "__ARM_FEATURE_ATOMICS", pfile);
   aarch64_def_or_undef (TARGET_AES, "__ARM_FEATURE_AES", pfile);
   aarch64_def_or_undef (TARGET_SHA2, "__ARM_FEATURE_SHA2", pfile);
   aarch64_def_or_undef (TARGET_SHA3, "__ARM_FEATURE_SHA3", pfile);