[Patch AArch64] Add __ARM_FEATURE_ATOMICS

[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.

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

From-SVN: r270770