runtime: use monotonic time on NetBSD

Fixes #6007

Change-Id: I239a1699122e086e907ac1f18b1c86a650e1438a
Reviewed-on: https://go-review.googlesource.com/81135
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/sys_netbsd_386.s

@@ -164,7 +164,7 @@ TEXT runtime·walltime(SB), NOSPLIT, $32
 // void nanotime(int64 *nsec)
 TEXT runtime·nanotime(SB),NOSPLIT,$32
 	LEAL	12(SP), BX
-	MOVL	$0, 4(SP)		// arg 1 - clock_id
+	MOVL	$3, 4(SP)		// arg 1 - clock_id CLOCK_MONOTONIC
 	MOVL	BX, 8(SP)		// arg 2 - tp
 	MOVL	$427, AX		// sys_clock_gettime
 	INT	$0x80

src/runtime/sys_netbsd_amd64.s

@@ -188,7 +188,7 @@ TEXT runtime·walltime(SB), NOSPLIT, $32
 	RET
 
 TEXT runtime·nanotime(SB),NOSPLIT,$32
-	MOVQ	$0, DI			// arg 1 - clock_id
+	MOVQ	$3, DI			// arg 1 - clock_id CLOCK_MONOTONIC
 	LEAQ	8(SP), SI		// arg 2 - tp
 	MOVL	$427, AX		// sys_clock_gettime
 	SYSCALL