[NFC][libcxxabi] Rename GlobalLock to GlobalMutex

This will make the naming more consistent with what it's called in the
rest of the file.

This is the 1st of 5 changes to overhaul cxa_guard.
See D108343 for what the final result will be.

Reviewed By: ldionne, #libc_abi

Differential Revision: https://reviews.llvm.org/D109539