cmd/internal/obj: clean up handling of register list operand on ARM

cmd/internal/obj: clean up handling of register list operand on ARM

ARM operands for MOVM have lists of registers: [R1,R2,R5-R8].
Handle them cleanly.

It was TYPE_CONST with special handling, which meant operand printing
didn't work right and the special handling was ugly. Add a new TYPE_REGLIST
for this case and it all gets cleaner.

Change-Id: I4a64f70fb9765e63cb636619a7a8553611bfe970
Reviewed-on: https://go-review.googlesource.com/6300
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>