runtime: use monotonic clock for timers on dragonfly.

Update #6007

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/69040045

src/pkg/runtime/sys_dragonfly_386.s

@@ -155,7 +155,7 @@ 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
@@ -172,7 +172,7 @@ TEXT time·now(SB), NOSPLIT, $32
 TEXT runtime·nanotime(SB), NOSPLIT, $32
 	MOVL	$232, AX
 	LEAL	12(SP), BX
-	MOVL	$0, 4(SP)
+	MOVL	$4, 4(SP)	// CLOCK_MONOTONIC
 	MOVL	BX, 8(SP)
 	INT	$0x80
 	MOVL	12(SP), AX	// sec

src/pkg/runtime/sys_dragonfly_amd64.s

@@ -125,7 +125,7 @@ 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
@@ -138,7 +138,7 @@ TEXT time·now(SB), NOSPLIT, $32
 
 TEXT runtime·nanotime(SB), NOSPLIT, $32
 	MOVL	$232, AX
-	MOVQ	$0, DI
+	MOVQ	$4, DI  	// CLOCK_MONOTONIC
 	LEAQ	8(SP), SI
 	SYSCALL
 	MOVQ	8(SP), AX	// sec