[Sanitizer][Darwin] Factor out code for GCD worker registration

This is a NFC change to factor out GCD worker thread registration via
the pthread introspection hook.

In a follow-up change we also want to register GCD workers for ASan to
make sure threads are registered before we attempt to print reports on
them.

rdar://93276353

Differential Revision: https://reviews.llvm.org/D126351