IBM Z: Fix testsuite useable_hw check

IBM Z: Fix testsuite useable_hw check

This fixes various issues with the useable_hw check in s390.exp.  The
check is supposed to verify whether a testcase can be run on the
current hardware.

- the test never returned true for -m31 because vzero is not available
  in ESA mode and -m31 defaults to -mesa
- the missing v0 clobber on the vzero instruction made the check fail
  if the stack pointer got saved in f0
- the lcbb instruction used for checking whether we are on a z13
  also requires vx.  Replace it with an instruction from the generic
  instruction set extensions.
- no support for z14 and z15 so far

gcc/testsuite/ChangeLog:

2019-11-05  Andreas Krebbel  <krebbel@linux.ibm.com>

	* gcc.target/s390/s390.exp
	(check_effective_target_s390_useable_hw): Add inline asm for z14
	and z15. Replace instruction for z13 with lochiz. Add register
	clobbers. Check also for __zarch__ when doing the __VX__ test.

From-SVN: r277830