[lsan] Factor pthread-specific assumptions out of thread tracking code

This is a small refactoring to prepare for porting LSan to Fuchsia.
Factor out parts of lsan_thread.{cpp,h} that don't apply to Fuchsia.
Since existing supported systems are POSIX-based, the affected code
is moved to lsan_posix.{cpp.h}.

Patch By: mcgrathr

Differential Revision: https://reviews.llvm.org/D73309