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.
