cmd/internal/obj/s390x: handle RestArgs in s390x assembler

Allow up to 3 RestArgs arguments to be specified. This is needed to
for us to add the 'rotate and ... bits' instructions, which require
5 arguments, cleanly.

Change-Id: I76b89adfb5e3cd85a43023e412f0cc202d489e0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/171726
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>