MFC: r234348

MFC: r234348

Turn on PREEMPTION by default. After fixing several bugs over time, the
last show-stopper keeping PREEMPTION from being usable on sparc64 should
have been dealt with in r230662 (MFC'ed to stable/9 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.

git-svn-id: svn://svn.freebsd.org/base/stable/9@235992 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f