cpu: also use MRS instruction in getmmfr1

Use the MRS instruction with the corresponding system register instead
of encoding the instructions using a WORD directive. This was missed in
CL 583135.

Change-Id: Ifcbbd81baa903f1da442eae2e49c06284db9c37d
Reviewed-on: https://go-review.googlesource.com/c/sys/+/717681
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>