s390-vregs.exp: Avoid compile errors with older GCCs and on 31-bit targets

s390-vregs.exp: Avoid compile errors with older GCCs and on 31-bit targets

The test case s390-vregs.exp yields compile errors on 31-bit targets
as well as when using a GCC that defaults to an older "-march=".  This
patch fixes these issues.

gdb/testsuite/ChangeLog:

	* gdb.arch/s390-vregs.S (change_vrs): Replace exrl by an
	appropriate .insn, such that an older assembler can be used.
	* gdb.arch/s390-vregs.exp: Add the compile flag -mzarch, to enable
	the z/Architecture instruction set on 31-bit targets as well.