Force -m31/-m64 based on s390/s390x target.

Force -march=z990.  The TCG backend will always require the
long-displacement facility, so the compiler may as well make
use of that as well.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>