[MachCore] Report arm64 thread exception state

A MachO userspace corefile may contain LC_THREAD commands which specify
thread exception state.

For arm64* only (for now), report a human-readable version of this state
as the thread stop reason, instead of 'SIGSTOP'.

As a follow-up, similar functionality can be implemented for x86 cores
by translating the trapno/err exception registers.

rdar://82898146

Differential Revision: https://reviews.llvm.org/D109795