[Patch AArch64] Add __ARM_FEATURE_ATOMICS

[Ramana Radhakrishnan]

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.


regards
Ramana

* config/aarch64/aarch64-c.c (aarch64_update_cpp_builtins): Define 
__ARM_FEATURE_ATOMICS
diff --git a/gcc/config/aarch64/aarch64-c.c b/gcc/config/aarch64/aarch64-c.c
index fcb1e80177d..6d5acb02fc6 100644
--- a/gcc/config/aarch64/aarch64-c.c
+++ b/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);