[dev.ssa] runtime: fix windows build

Pull the relevant part of https://go-review.googlesource.com/#/c/18304/
into the SSA branch.  This fixes the windows SSA build.

Change-Id: Iee3834d8e7019cd90307e32029f3d1032ceb46c1
Reviewed-on: https://go-review.googlesource.com/18673Reviewed-by: Austin Clements <austin@google.com>

src/runtime/sys_windows_amd64.s

@@ -243,7 +243,7 @@ TEXT runtime·externalthreadhandler(SB),NOSPLIT,$0
 	LEAQ	g__size(SP), BX
 	MOVQ	BX, g_m(SP)
 
-	LEAQ	-8192(SP), CX
+	LEAQ	-32768(SP), CX
 	MOVQ	CX, (g_stack+stack_lo)(SP)
 	ADDQ	$const__StackGuard, CX
 	MOVQ	CX, g_stackguard0(SP)