[OpenMP] Cleanup synchronization primitives (#177710)

Summary:
These shouldn't be so different after we moved away from variants. It's
much simpler to define this in-line with a single preprocessor
definition. This should be equivalent less a few unnecessary function
definitions with the advantage that SPIR-V now has less work to do.
GitOrigin-RevId: 55979032401ebdb62085f677770fc6c5793b584e