runtime: use clock_gettime(CLOCK_REALTIME) for nanosecond-precision time.now on arm64, mips64x

Assembly copied from the clock_gettime(CLOCK_MONOTONIC)
call in runtime.nanotime in these files and then modified to use
CLOCK_REALTIME.

Also comment system call numbers in a few other files.

Fixes #11222.

Change-Id: Ie132086de7386f865908183aac2713f90fc73e0d
Reviewed-on: https://go-review.googlesource.com/32177Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/sys_darwin_386.s

@@ -200,7 +200,7 @@ systime:
 	MOVL	AX, 4(SP)
 	MOVL	$0, 8(SP)	// time zone pointer
 	MOVL	$0, 12(SP)	// required as of Sierra; Issue 16570
-	MOVL	$116, AX
+	MOVL	$116, AX // SYS_GETTIMEOFDAY
 	INT	$0x80
 	CMPL	AX, $0
 	JNE	inreg

src/runtime/sys_darwin_amd64.s

@@ -158,7 +158,7 @@ systime:
 	MOVQ	SP, DI
 	MOVQ	$0, SI
 	MOVQ	$0, DX  // required as of Sierra; Issue 16570
-	MOVL	$(0x2000000+116), AX
+	MOVL	$(0x2000000+116), AX // gettimeofday
 	SYSCALL
 	CMPQ	AX, $0
 	JNE	inreg

src/runtime/sys_dragonfly_amd64.s

@@ -150,7 +150,7 @@ TEXT runtime·setitimer(SB), NOSPLIT, $-8
 
 // func now() (sec int64, nsec int32)
 TEXT time·now(SB), NOSPLIT, $32
-	MOVL	$232, AX
+	MOVL	$232, AX // clock_gettime
 	MOVQ	$0, DI  	// CLOCK_REALTIME
 	LEAQ	8(SP), SI
 	SYSCALL

src/runtime/sys_freebsd_386.s

@@ -161,7 +161,7 @@ TEXT runtime·setitimer(SB), NOSPLIT, $-4
 
 // func now() (sec int64, nsec int32)
 TEXT time·now(SB), NOSPLIT, $32
-	MOVL	$232, AX
+	MOVL	$232, AX // clock_gettime
 	LEAL	12(SP), BX
 	MOVL	$0, 4(SP)	// CLOCK_REALTIME
 	MOVL	BX, 8(SP)

src/runtime/sys_freebsd_amd64.s

@@ -144,7 +144,7 @@ TEXT runtime·setitimer(SB), NOSPLIT, $-8
 
 // func now() (sec int64, nsec int32)
 TEXT time·now(SB), NOSPLIT, $32
-	MOVL	$232, AX
+	MOVL	$232, AX // clock_gettime
 	MOVQ	$0, DI		// CLOCK_REALTIME
 	LEAQ	8(SP), SI
 	SYSCALL

src/runtime/sys_linux_arm64.s

@@ -184,14 +184,12 @@ TEXT runtime·mincore(SB),NOSPLIT,$-8-28
 
 // func now() (sec int64, nsec int32)
 TEXT time·now(SB),NOSPLIT,$24-12
-	MOVD	RSP, R0
-	MOVD	$0, R1
-	MOVD	$SYS_gettimeofday, R8
+	MOVW	$0, R0 // CLOCK_REALTIME
+	MOVD	RSP, R1
+	MOVD	$SYS_clock_gettime, R8
 	SVC
 	MOVD	0(RSP), R3	// sec
-	MOVD	8(RSP), R5	// usec
-	MOVD	$1000, R4
-	MUL	R4, R5
+	MOVD	8(RSP), R5	// nsec
 	MOVD	R3, sec+0(FP)
 	MOVW	R5, nsec+8(FP)
 	RET

src/runtime/sys_linux_mips64x.s

@@ -174,15 +174,12 @@ TEXT runtime·mincore(SB),NOSPLIT,$-8-28
 
 // func now() (sec int64, nsec int32)
 TEXT time·now(SB),NOSPLIT,$16
-	MOVV	$0(R29), R4
-	MOVV	$0, R5
-	MOVV	$SYS_gettimeofday, R2
+	MOVW	$0, R4 // CLOCK_REALTIME
+	MOVV	$0(R29), R5
+	MOVV	$SYS_clock_gettime, R2
 	SYSCALL
 	MOVV	0(R29), R3	// sec
-	MOVV	8(R29), R5	// usec
-	MOVV	$1000, R4
-	MULVU	R4, R5
-	MOVV	LO, R5
+	MOVV	8(R29), R5	// nsec
 	MOVV	R3, sec+0(FP)
 	MOVW	R5, nsec+8(FP)
 	RET