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>

cpu/cpu_arm64.s

diff --git a/cpu/cpu_arm64.s b/cpu/cpu_arm64.s
index 601a5f7..a4f24b3 100644
--- a/cpu/cpu_arm64.s
+++ b/cpu/cpu_arm64.s
@@ -23,8 +23,7 @@
 // func getmmfr1() uint64
 TEXT ·getmmfr1(SB),NOSPLIT,$0-8
 	// get Memory Model Feature Register 1 into x0
-	// mrs	x0, ID_AA64MMFR1_EL1 = d5380720
-	WORD $0xd5380720
+	MRS	ID_AA64MMFR1_EL1, R0
 	MOVD	R0, ret+0(FP)
 	RET