Merge libcxxrt commit f2e55091e2e878386c9f7974d4922bbdc4faed84

Fix unlock in two-word version and add missing comment.

Fixes #15
Fixes #16

This should fix the hangs in __cxa_guard_acquire() reported on i386 (and
possibly other 32-bit platforms).

Obtained from: https://github.com/libcxxrt/libcxxrt/commit/f2e5509
Fixes: 56aaed388b0a
MFC after: 2 weeks

(cherry picked from commit 7819a911ff5132b34996c8b09b2a024cd10fd4fb)