<utils/Macros.h>: introduce

Summary:
Issue:
These macros are non-standard to the C standard, but standard in GCC.
This causes issues since compiler-rt assumes we provide these when built with
LLVM Clang.

Solution:
Include these macros as a easy to utilize header.

Test Plan: verification is a WIP

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D107