As with the previous round for ppc64, this CL fixes a couple of things
that 5a supported but asm did not, both simple.

1) Allow condition code on MRC instruction; this was marked as a TODO.
2) Allow R(n) notation in ARM register shifts.  The code needs a rethink
but the tests we're leading toward will make the rewrite easier to test and
trust.

Change-Id: I5b52ad25d177a74cf07e089dddfeeab21863c424
Reviewed-on: https://go-review.googlesource.com/5422Reviewed-by: Russ Cox <rsc@golang.org>