[libc++] Refactoring __sync_* builtins; NFC

Summary: Wrap __sync_* builtins with __libcpp_ functions to facility future customizations as atomic operations are unavailable on some targets.

Reviewers: danalbert, EricWF, jroelofs

Subscribers: joerg, llvm-commits

Differential Revision: https://reviews.llvm.org/D34918

llvm-svn: 307591