Revert "[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.
Reviewed By: delcypher, yln
Differential Revision: https://reviews.llvm.org/D97509
This reverts commit 71ef54337d5e83c6767e4816227ef340caa337fc.