cmd/compile/internal/s390x: replace 4-byte NOP with a 2-byte NOP on s390x

cmd/compile/internal/s390x: replace 4-byte NOP with a 2-byte NOP on s390x

Added a new instruction, NOPH, with the encoding [0x0700](i.e: bcr 0, 0) and
replace the current 4-byte nop that was encoded using the WORD instruction.

This reduces the size of .text section in go binary by around 17KB and make
generated code easier to read.

Change-Id: I6a756df39e93c4415ea6d038ba4af001b8ccb286
Reviewed-on: https://go-review.googlesource.com/c/go/+/194344
Reviewed-by: Michael Munday <mike.munday@ibm.com>
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>