runtime: fix nanotime for macOS Sierra, again.

This is a cherry-pick of https://go-review.googlesource.com/25400
to the release-branch-go1.4

macOS Sierra beta4 changed the kernel interface for getting time.
DX now optionally points to an address for additional info.
Set it to zero to avoid corrupting memory.

Fixes golang#16570

Change-Id: I714325a7749a145d23cf03251db38196ac9c481a
Reviewed-on: https://go-review.googlesource.com/31750
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Author: josharian

src/runtime/sys_darwin_386.s

@@ -182,15 +182,16 @@ timeloop:
 
 systime:
 	// Fall back to system call (usually first call in this thread)
-	LEAL	12(SP), AX	// must be non-nil, unused
+	LEAL	16(SP), AX	// must be non-nil, unused
 	MOVL	AX, 4(SP)
 	MOVL	$0, 8(SP)	// time zone pointer
+	MOVL	$0, 12(SP)	// required as of Sierra; Issue 16570
 	MOVL	$116, AX
 	INT	$0x80
 	CMPL	AX, $0
 	JNE	inreg
-	MOVL	12(SP), AX
-	MOVL	16(SP), DX
+	MOVL	16(SP), AX
+	MOVL	20(SP), DX
 inreg:
 	// sec is in AX, usec in DX
 	// convert to DX:AX nsec

src/runtime/sys_darwin_amd64.s

@@ -143,6 +143,7 @@ systime:
 	// Fall back to system call (usually first call in this thread).
 	MOVQ	SP, DI
 	MOVQ	$0, SI
+	MOVQ	$0, DX  // required as of Sierra; Issue 16570
 	MOVL	$(0x2000000+116), AX
 	SYSCALL
 	CMPQ	AX, $0