s390: Assemble processor specific test cases for their processor

Assemble the esa-g5 test case with -march=g5.
Assemble the zarch-z900 test case with -march=z900.

gas/
	* testsuite/gas/s390/s390.exp: Assemble processor specific test
	cases for their respective processor (-march=<processor>).

Reviewed-by: Andreas Krebbel <krebbel@linux.ibm.com>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>