Add __builtin___clear_cache to the portability headers

Add __builtin___clear_cache to the portability headers

Summary:
HHVM uses this heavily in it's ARM JIT, so provide an implementation. With GCC & Clang this function is a no-op on x86_64 where the requirement doesn't exist, so do the same in the portability header.
This doesn't include the Windows portability header on x64 specifically because it is a very heavy header and this is currently very lightweight.

Reviewed By: yfeldblum

Differential Revision: D4451032

fbshipit-source-id: 0af57448577635f9cd690ea9b09ce46a244191e1