Rewrite cxa guard implementation.
This patch does three main things:
(1) It re-writes the cxa guard implementation to make it testable. (2) Adds support for recursive init detection on non-apple platforms. (3) It adds a futex based implementation.
The futex based implementation locks and notifies on a per-object basis, unlike the
current implementation which uses a global lock for all objects. Once this patch settles
I'll turn it on by default when supported.