cmd/compile/internal/gc: refactor cgen_div

cmd/compile/internal/gc: refactor cgen_div

This commit adds two new functions to cgen.go: hasHMUL64 and
hasRROTC64. These are used to determine whether or not an
architecture supports the instructions needed to perform an
optimization in cgen_div.

This commit should not affect existing architectures (although it
does add s390x to the new functions). However, since most
architectures support HMUL the hasHMUL64 function could be
modified to enable most of the optimizations in cgen_div on those
platforms.

Change-Id: I33bf329ddeb6cf2954bd17b7c161012de352fb62
Reviewed-on: https://go-review.googlesource.com/21775
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>