The old, per-architecture operand printers didn't lock down the
format of the constant in the MRC and MCR instructions (a value
that could be presented more helpfully - maybe how the
input looks? - but that is an issue for another day). But there is
a portable standard printer now so we can enable tests for these
instructions.

Change-Id: I437a3b112ce63f4d6e1fe3450fc21d8c3372602f
Reviewed-on: https://go-review.googlesource.com/7420Reviewed-by: Russ Cox <rsc@golang.org>