sanitizer_common: replace RWMutex/BlockingMutex with Mutex

Mutex supports reader access, OS blocking, spinning,
portable and smaller than BlockingMutex.
Overall it's supposed to be better than RWMutex/BlockingMutex.
Replace RWMutex/BlockingMutex with Mutex.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D106936