[release-branch.go1.5] cmd/asm: handle CMPF and CMPD on ARM

These instructions are special cases that were missed in the translation.
The second argument must go into the Reg field not the To field.

Fixes #12458

For Go 1.5.1

Change-Id: Iad57c60c7e38e3bcfafda483ed5037ce670e8816
Reviewed-on: https://go-review.googlesource.com/14183


Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-on: https://go-review.googlesource.com/14358


Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Rob Pike <r@golang.org>