Repeat the spinlock_enter/exit pattern from amd64 on other architectures to

Repeat the spinlock_enter/exit pattern from amd64 on other architectures to
fix an assert violation introduced in r355784.  Without this spinlock_exit()
may see owepreempt and switch before reducing the spinlock count.  amd64
had been optimized to do a single critical enter/exit regardless of the
number of spinlocks which avoided the problem and this optimization had
not been applied elsewhere.

Reported by:	emaste
Suggested by:	rlibby
Discussed with:	jhb, rlibby
Tested by:	manu (arm64)