runtime: fix nanotime for macOS Sierra, again.

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 #16570

Change-Id: I9f537e552682045325cdbb68b7d0b4ddafade14a
Reviewed-on: https://go-review.googlesource.com/25400
Reviewed-by: David Crawshaw <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Quentin Smith <[email protected]>
Run-TryBot: Brad Fitzpatrick <[email protected]>
Reviewed-by: Austin Clements <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: bradfitz

src/runtime/sys_darwin_386.s

@@ -196,15 +196,16 @@ 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

@@ -157,6 +157,7 @@ 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