runtime: fix windows/386 build

The difference between the old and the new (from earlier) code
is that we set stackguard = stack.lo + StackGuard, while the old
code set stackguard = stack.lo. That 512 bytes appears to be
the difference between the profileloop function running and not running.

We don't know how big the system stack is, but it is likely MUCH bigger than 4k.
Give Go/C 8k.

TBR=iant
CC=golang-codereviews
https://golang.org/cl/140440044

src/runtime/memclr_386.s

@@ -6,6 +6,8 @@
 
 #include "textflag.h"
 
+// NOTE: Windows externalthreadhandler expects memclr to preserve DX.
+
 // void runtime·memclr(void*, uintptr)
 TEXT runtime·memclr(SB), NOSPLIT, $0-8
 	MOVL	ptr+0(FP), DI

src/runtime/memclr_amd64.s

@@ -6,6 +6,8 @@
 
 #include "textflag.h"
 
+// NOTE: Windows externalthreadhandler expects memclr to preserve DX.
+
 // void runtime·memclr(void*, uintptr)
 TEXT runtime·memclr(SB), NOSPLIT, $0-16
 	MOVQ	ptr+0(FP), DI

src/runtime/sys_windows_386.s

@@ -191,7 +191,7 @@ TEXT runtime·externalthreadhandler(SB),NOSPLIT,$0
 	CALL	runtime·memclr(SB)	// smashes AX,BX,CX
 	LEAL	g_end(SP), BX
 	MOVL	BX, g_m(SP)
-	LEAL	-4096(SP), CX
+	LEAL	-8192(SP), CX
 	MOVL	CX, (g_stack+stack_lo)(SP)
 	ADDL	$const_StackGuard, CX
 	MOVL	CX, g_stackguard0(SP)