Apply r355819 to sparc64 - fix assertion failure after r355784

Apply r355819 to sparc64 - fix assertion failure after r355784

From r355819:
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.

This is completely untested - I have no obsolete Sparc hardware - but
someone did try testing recent changes on sparc64 (PR 243534).

PR:		243534