[sanitizer] Pair atomic acquire with release in BlockingMutex::Unlock

Summary:
Dmitry, seeking your expertise. I believe, the proper way to implement
Lock/Unlock here would be to use acquire/release semantics. Am I missing
something?

Reviewers: dvyukov

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D33521

llvm-svn: 303869