8l,6l: allow for more os threads to be created on Windows

Program listed http://code.google.com/p/go/issues/detail?id=1495
(with nRequester set to 10000) will crash with

runtime: failed to create new OS thread (have 4526 already; errno=8)

instead of

runtime: failed to create new OS thread (have 618 already; errno=8).

R=golang-dev, rsc, vcc
CC=golang-dev
https://golang.org/cl/4172046

src/cmd/ld/pe.c

@@ -526,7 +526,7 @@ asmbpe(void)
 	set(SizeOfImage, nextsectoff);
 	set(SizeOfHeaders, PEFILEHEADR);
 	set(Subsystem, 3);	// WINDOWS_CUI
-	set(SizeOfStackReserve, 0x00200000);
+	set(SizeOfStackReserve, 0x0040000);
 	set(SizeOfStackCommit, 0x00001000);
 	set(SizeOfHeapReserve, 0x00100000);
 	set(SizeOfHeapCommit, 0x00001000);