runtime: use monotonic clock for timers on freebsd/amd64,386,arm

For now we don't use CLOCK_MONOTONIC_FAST instead because
it's not supported on prior to 9-STABLE.

Update #6007

LGTM=minux.ma
R=golang-codereviews, minux.ma, bradfitz
CC=golang-codereviews
https://golang.org/cl/68690043

src/pkg/runtime/sys_freebsd_386.s

@@ -135,7 +135,7 @@ TEXT runtime·setitimer(SB), NOSPLIT, $-4
 TEXT time·now(SB), NOSPLIT, $32
 	MOVL	$232, AX
 	LEAL	12(SP), BX
-	MOVL	$0, 4(SP)
+	MOVL	$0, 4(SP)	// CLOCK_REALTIME
 	MOVL	BX, 8(SP)
 	INT	$0x80
 	MOVL	12(SP), AX	// sec
@@ -152,7 +152,9 @@ TEXT time·now(SB), NOSPLIT, $32
 TEXT runtime·nanotime(SB), NOSPLIT, $32
 	MOVL	$232, AX
 	LEAL	12(SP), BX
-	MOVL	$0, 4(SP)
+	// We can use CLOCK_MONOTONIC_FAST here when we drop
+	// support for FreeBSD 8-STABLE.
+	MOVL	$4, 4(SP)	// CLOCK_MONOTONIC
 	MOVL	BX, 8(SP)
 	INT	$0x80
 	MOVL	12(SP), AX	// sec

src/pkg/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
-	MOVQ	$0, DI
+	MOVQ	$0, DI		// CLOCK_REALTIME
 	LEAQ	8(SP), SI
 	SYSCALL
 	MOVQ	8(SP), AX	// sec
@@ -157,7 +157,9 @@ TEXT time·now(SB), NOSPLIT, $32
 
 TEXT runtime·nanotime(SB), NOSPLIT, $32
 	MOVL	$232, AX
-	MOVQ	$0, DI
+	// We can use CLOCK_MONOTONIC_FAST here when we drop
+	// support for FreeBSD 8-STABLE.
+	MOVQ	$4, DI		// CLOCK_MONOTONIC
 	LEAQ	8(SP), SI
 	SYSCALL
 	MOVQ	8(SP), AX	// sec

src/pkg/runtime/sys_freebsd_arm.s

@@ -162,7 +162,9 @@ TEXT time·now(SB), NOSPLIT, $32
 // int64 nanotime(void) so really
 // void nanotime(int64 *nsec)
 TEXT runtime·nanotime(SB), NOSPLIT, $32
-	MOVW $0, R0 // CLOCK_REALTIME
+	// We can use CLOCK_MONOTONIC_FAST here when we drop
+	// support for FreeBSD 8-STABLE.
+	MOVW $4, R0 // CLOCK_MONOTONIC
 	MOVW $8(R13), R1
 	MOVW $SYS_clock_gettime, R7
 	SWI $0