[XRay][compiler-rt] Use sanitizer_common's atomic ops

[XRay][compiler-rt] Use sanitizer_common's atomic ops

Instead of std::atomic APIs for atomic operations, we instead use APIs
include with sanitizer_common. This allows us to, at runtime, not have
to depend on potentially dynamically provided implementations of these
atomic operations.

Fixes http://llvm.org/PR32274.

llvm-svn: 298833