cpu: use MRS instruction to read arm64 system registers

Use the MRS instruction with the corresponding system register instead
of encoding the instructions using a WORD directive.

Change-Id: I2995dfa6ad731cb03867160127db84898adfdda5
Reviewed-on: https://go-review.googlesource.com/c/sys/+/583135
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>

cpu/cpu_arm64.s

@@ -9,16 +9,14 @@
 // func getisar0() uint64
 TEXT ·getisar0(SB),NOSPLIT,$0-8
 	// get Instruction Set Attributes 0 into x0
-	// mrs x0, ID_AA64ISAR0_EL1 = d5380600
+	MRS	ID_AA64ISAR0_EL1, R0
-	WORD	$0xd5380600
 	MOVD	R0, ret+0(FP)
 	RET
 
 // func getisar1() uint64
 TEXT ·getisar1(SB),NOSPLIT,$0-8
 	// get Instruction Set Attributes 1 into x0
-	// mrs x0, ID_AA64ISAR1_EL1 = d5380620
+	MRS	ID_AA64ISAR1_EL1, R0
-	WORD	$0xd5380620
 	MOVD	R0, ret+0(FP)
 	RET
 
@@ -33,15 +31,13 @@ TEXT ·getmmfr1(SB),NOSPLIT,$0-8
 // func getpfr0() uint64
 TEXT ·getpfr0(SB),NOSPLIT,$0-8
 	// get Processor Feature Register 0 into x0
-	// mrs x0, ID_AA64PFR0_EL1 = d5380400
+	MRS	ID_AA64PFR0_EL1, R0
-	WORD	$0xd5380400
 	MOVD	R0, ret+0(FP)
 	RET
 
 // func getzfr0() uint64
 TEXT ·getzfr0(SB),NOSPLIT,$0-8
 	// get SVE Feature Register 0 into x0
-	// mrs	x0, ID_AA64ZFR0_EL1 = d5380480
+	MRS	ID_AA64ZFR0_EL1, R0
-	WORD $0xd5380480
 	MOVD	R0, ret+0(FP)
 	RET