cmd/asm: add s390x 'rotate then ... selected bits' instructions

cmd/asm: add s390x 'rotate then ... selected bits'  instructions

This CL adds the following instructions, useful for shifting/rotating
and masking operations:

 * RNSBG - rotate then and selected bits
 * ROSBG - rotate then or selected bits
 * RXSBG - rotate then exclusive or selected bits
 * RISBG - rotate then insert selected bits

It also adds the 'T' (test), 'Z' (zero), 'H' (high), 'L' (low) and
'N' (no test) variants of these instructions as appropriate.

Operands are ordered as: I₃, I₄, I₅, R₂, R₁.

Key: I₃=start, I₄=end, I₅=amount, R₂=source, R₁=destination

Change-Id: I200d12287e1df7447f37f4919da5e9a93d27c792
Reviewed-on: https://go-review.googlesource.com/c/go/+/159357
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>