runtime: correct netbsd/amd64 assembly for timespec handling

A timespec on netbsd/amd64 is int64/int64, not int64/int32.
This bug appears to have been introduced in 7777bac6.

Spotted by Cherry Zhang while reviewing https://go-review.googlesource.com/c/go/+/177120.

Change-Id: I163c55d926965defd981bdbfd2511de7d9d4c542
Reviewed-on: https://go-review.googlesource.com/c/go/+/177637Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Benny Siegert <bsiegert@gmail.com>

src/runtime/sys_netbsd_amd64.s

@@ -218,7 +218,7 @@ TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVL	$SYS___clock_gettime50, AX
 	SYSCALL
 	MOVQ	8(SP), AX		// sec
-	MOVL	16(SP), DX		// nsec
+	MOVQ	16(SP), DX		// nsec
 
 	// sec is in AX, nsec in DX
 	MOVQ	AX, sec+0(FP)
@@ -231,7 +231,7 @@ TEXT runtime·nanotime(SB),NOSPLIT,$32
 	MOVL	$SYS___clock_gettime50, AX
 	SYSCALL
 	MOVQ	8(SP), AX		// sec
-	MOVL	16(SP), DX		// nsec
+	MOVQ	16(SP), DX		// nsec
 
 	// sec is in AX, nsec in DX
 	// return nsec in AX