[libcxxabi] Make InitByteGlobalMutex check GetThreadID instead of PlatformThreadID

By relying on PlatformSupportsThreadID, InitByteGlobalMutex disregards
the GetThreadID template argument, rendering it useless.

This is the 2nd of 5 changes to overhaul cxa_guard.
See D108343 for what the final result will be.

Depends on D109539

Reviewed By: ldionne, #libc_abi

Differential Revision: https://reviews.llvm.org/D110088