Switch to a .cpu directive. These will work when clang 3.5 is imported
where the .arch directive is a nop.

MFC after:	1 week
Sponsored by:	ABT Systems Ltd


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275524 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f