[darwin] switch blocking mutex from osspinlock to os_unfair_lock

OSSpinLock is deprecated, so we are switching to `os_unfair_lock`. However, `os_unfair_lock` isn't available on older OSs, so we keep `OSSpinLock` as fallback.

Also change runtime assumption check to static since they only ever check constant values.

rdar://69588111

Reviewed By: delcypher, yln

Differential Revision: https://reviews.llvm.org/D97509