runtime/cgo: correct type declaration for Windows

Newer versions of gcc notice a type mismatch and complain.
Fix code to match documented signature in MSDN.
Trybots say this still compiles with the older (5.1) version
of gcc.

Fixes #17771.

Change-Id: Ib3fe6f71b40751e1146249e31232da5ac69b9e00
Reviewed-on: https://go-review.googlesource.com/32646
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/cgo/gcc_libinit_windows.c

@@ -12,8 +12,8 @@
 
 #include "libcgo.h"
 
-static volatile long runtime_init_once_gate = 0;
-static volatile long runtime_init_once_done = 0;
+static volatile LONG runtime_init_once_gate = 0;
+static volatile LONG runtime_init_once_done = 0;
 
 static CRITICAL_SECTION runtime_init_cs;