runtime: use MRS instruction to read system registers on freebsd/arm64

Support for these was added in CL 189577

Change-Id: Iaf2a774b141995cbbdfb3888aea67ae9c7f928b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/210677
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/sys_freebsd_arm64.s

@@ -506,38 +506,33 @@ TEXT runtime·getCntxct(SB),NOSPLIT,$0
 	CMP	$0, R0
 	BEQ	3(PC)
 
-	// get CNTPCT (Physical Count Register) into x0
-	// mrs x0, cntpct_el0 = d53be020
-	WORD	$0xd53be020 // SIGILL
+	// get CNTPCT (Physical Count Register) into R0
+	MRS	CNTPCT_EL0, R0 // SIGILL
 	B	2(PC)
 
-	// get CNTVCT (Virtual Count Register) into x0
-	// mrs x0, cntvct_el0 = d53be040
-	WORD	$0xd53be040
+	// get CNTVCT (Virtual Count Register) into R0
+	MRS	CNTVCT_EL0, R0
 
 	MOVW	R0, ret+8(FP)
 	RET
 
 // func getisar0() uint64
 TEXT runtime·getisar0(SB),NOSPLIT,$0
-	// get Instruction Set Attributes 0 into x0
-	// mrs x0, ID_AA64ISAR0_EL1 = d5380600
-	WORD	$0xd5380600
+	// get Instruction Set Attributes 0 into R0
+	MRS	ID_AA64ISAR0_EL1, R0
 	MOVD	R0, ret+0(FP)
 	RET
 
 // func getisar1() uint64
 TEXT runtime·getisar1(SB),NOSPLIT,$0
-	// get Instruction Set Attributes 1 into x0
-	// mrs x0, ID_AA64ISAR1_EL1 = d5380620
-	WORD	$0xd5380620
+	// get Instruction Set Attributes 1 into R0
+	MRS	ID_AA64ISAR1_EL1, R0
 	MOVD	R0, ret+0(FP)
 	RET
 
 // func getpfr0() uint64
 TEXT runtime·getpfr0(SB),NOSPLIT,$0
-	// get Processor Feature Register 0 into x0
-	// mrs x0, ID_AA64PFR0_EL1 = d5380400
-	WORD	$0xd5380400
+	// get Processor Feature Register 0 into R0
+	MRS	ID_AA64PFR0_EL1, R0
 	MOVD	R0, ret+0(FP)
 	RET