[NFC][compiler-rt][hwasan] Refactor hwasan functions

This moves the implementations for HandleTagMismatch, __hwasan_tag_mismatch4,
and HwasanAtExit from hwasan_linux.cpp to hwasan.cpp and declares them in hwasan.h.
This way, calls to those functions can be shared with the fuchsia implementation
without duplicating code.

Differential Revision: https://reviews.llvm.org/D103562