tcg-s390: Don't force -march=z990

While we still require the LONG DISPLACEMENT facility, defaults
have moved on since then.  Don't override the system compiler,
whose default may be set to z9-109 or later.

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>