last show-stopper keeping PREEMPTION from being usable on sparc64 should
have been dealt with in r230662.
At least on 2-way systems, PREEMPTION causes a little bit of a degradation
in worldstone performance. However, FreeBSD seems to have started building
up regressions in !PREEMPTION cases so sparc64 better should not be an
oddball in this regard.
MFC after: 1 week