cmd/internal: add MVCIN instruction to s390x assembler

On s390x, we already have MVCIN opcode in asmz.go,
but we did not use it. This CL uses that opcode and adds MVCIN
instruction.

MVCIN instruction can be used to move data from one storage location
to another while reversing the order of bytes within the field. This
could be useful when transforming data from little-endian to big-endian.

Change-Id: Ifa1a911c0d3442f4a62f91f74ed25b196d01636b
Reviewed-on: https://go-review.googlesource.com/c/go/+/227478


Reviewed-by: Michael Munday <mike.munday@ibm.com>
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>