cmd/asm, cmd/internal/obj/s390x, math: add "test under mask" instructions

cmd/asm, cmd/internal/obj/s390x, math: add "test under mask" instructions

Adds the following s390x test under mask (immediate) instructions:

TMHH
TMHL
TMLH
TMLL

These are useful for testing bits and are already used in the math package.

Change-Id: Idffb3f83b238dba76ac1e42ac6b0bf7f1d11bea2
Reviewed-on: https://go-review.googlesource.com/41092
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>