bsd-user: Stop building the sparc targets

The forked bsd-user tree doesn't really support these targets.  They aren't
functional at the moment anyway. Remove them from the build so that the major
reorg patch series can focus on one platform (x86) before adding addition
platforms once things are shuffled. This should make it easier to review and
satisfy that all revisions of a patch series are buildable.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>