Properly handle prefetch only for amd64 and i386 as we do elsewhere.

In general theraven is right that we should factr this out and provide
a general and per-arch implementation that everything can use.

MFC after:	3 days
X-MFC with:	r270755


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270806 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f