Add an Atomic portability header

Add an Atomic portability header

Summary: Because there are situations where we need to do an atomic write to a plain pointer, rather than an atomic value. Unfortunately, there is no support in the standard library for this, so different compilers implement the primitives differently. This specifically implements an `int64_t` overload for `__sync_fetch_and_add` for use in the atomic hash map test.

Reviewed By: yfeldblum

Differential Revision: D3571830

fbshipit-source-id: c27d8d2a5238bbc9aba6a9e48e4b3412a199288f