sanitizer_common: add Semaphore

Semaphore is a portable way to park/unpark threads.
The plan is to use it to implement a portable blocking
mutex in subsequent changes. Semaphore can also be used
to efficiently wait for other things (e.g. we currently
spin to synchronize thread creation and start).

Reviewed By: vitalybuka, melver

Differential Revision: https://reviews.llvm.org/D106071