[libc] Add a class "Atomic" as a simple equivalent of std::atomic.

Only the methods currently required by the libc have been added.
Most of the existing uses of atomic operations have been switched over
to this new class. A future change will clean up the rest of uses.

This change now allows building mutex and condition variable code with a
C++ compiler which does not have stdatomic.h, for example g++.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D120642