[Libomptarget] Introduce new main thread ID runtime function

This patch introduces `__kmpc_is_generic_main_thread_id` which splits the old
comparison into its own runtime function. The purpose of this is so we can fold
this part independently, so when both this and `is_spmd_mode` are folded the
final function will be folded as well.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D106437