Bug 1784018 - Remove deprecated OSSpinLocks r=glandium

On macOS versions prior to 10.15 os_unfair_locks cannot spin in kernel-space
which degrades performance significantly. To obviate for this we spin in
user-space like OSSpinLock does, for the same number of times and invoking
x86-specific pause instructions in-between the locking attempts to avoid
starving a thread that might be running on the same physical core.

Differential Revision: https://phabricator.services.mozilla.com/D154205