cmd/asm, cmd/internal/obj/s390x: add new s390x instructions for SSA

cmd/asm, cmd/internal/obj/s390x: add new s390x instructions for SSA

This commit adds the following instructions to support the new SSA
backend for s390x:

32-bit operations:
ADDW
SUBW
NEGW
FNEGS

Conditional moves:
MOVDEQ
MOVDGE
MOVDGT
MOVDLE
MOVDLT
MOVDNE

Unordered branches (for floating point comparisons):
BLEU
BLTU

Modulo operations:
MODW
MODWU
MODD
MODDU

The modulo operations might be removed in a future commit because
I'd like to change DIV to produce a tuple once the old backend is
removed.

This commit also removes uses of REGZERO from the assembler. They
aren't necessary and R0 will be used as a GPR by SSA.

Change-Id: I05756c1cbb74bf4a35fc492f8f0cd34b50763dc9
Reviewed-on: https://go-review.googlesource.com/29075
Run-TryBot: Michael Munday <munday@ca.ibm.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>