runtime: fix nanotime for macOS Sierra

In the beta version of the macOS Sierra (10.12) release, the
gettimeofday system call changed on x86. Previously it always returned
the time in the AX/DX registers. Now, if AX is returned as 0, it means
that the system call has stored the values into the memory pointed to by
the first argument, just as the libc gettimeofday function does. The
libc function handles both cases, and we need to do so as well.

Fixes #16272.

Change-Id: Ibe5ad50a2c5b125e92b5a4e787db4b5179f6b723
Reviewed-on: https://go-review.googlesource.com/24812
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/sys_darwin_386.s

@@ -201,6 +201,11 @@ systime:
 	MOVL	$0, 8(SP)	// time zone pointer
 	MOVL	$116, AX
 	INT	$0x80
+	CMPL	AX, $0
+	JNE	inreg
+	MOVL	12(SP), AX
+	MOVL	16(SP), DX
+inreg:
 	// sec is in AX, usec in DX
 	// convert to DX:AX nsec
 	MOVL	DX, BX

src/runtime/sys_darwin_amd64.s

@@ -155,10 +155,15 @@ timeloop:
 
 systime:
 	// Fall back to system call (usually first call in this thread).
-	MOVQ	SP, DI	// must be non-nil, unused
+	MOVQ	SP, DI
 	MOVQ	$0, SI
 	MOVL	$(0x2000000+116), AX
 	SYSCALL
+	CMPQ	AX, $0
+	JNE	inreg
+	MOVQ	0(SP), AX
+	MOVL	8(SP), DX
+inreg:
 	// sec is in AX, usec in DX
 	// return nsec in AX
 	IMULQ	$1000000000, AX