Fix problems in x64 build that were discovered by the testsuite:
- Reenable modules on x64 that had been disabled aeons ago for Itanium.
- Cleared up confusion about compilers for 64 bit windows.  There is only Itanium and x64.  Added macros MS_WINI64 and MS_WINX64 for those rare cases where it matters, such as the disabling of modules above.
- Set target platform (_WIN32_WINNT and WINVER) to 0x0501 (XP) for x64, and 0x0400 (NT 4.0) otherwise, which are the targeted minimum platforms.
- Fixed thread_nt.h.  The emulated InterlockedCompareExchange function didn´t work on x64, probaby due to the lack of a "volatile" specifier.  Anyway, win95 is no longer a target platform.
- Itertools module used wrong constant to check for overflow in count()
- PyInt_AsSsize_t couldn't deal with attribute error when accessing the __long__ member.
- PyLong_FromSsize_t() incorrectly specified that the operand were unsigned.

With these changes, the x64 passes the testsuite, for those modules present.
diff --git a/PC/config.c b/PC/config.c
index e1b52a4..6860211 100644
--- a/PC/config.c
+++ b/PC/config.c
@@ -6,21 +6,21 @@
 #include "Python.h"
 
 extern void initarray(void);
-#ifndef MS_WIN64
+#ifndef MS_WINI64
 extern void initaudioop(void);
 #endif
 extern void initbinascii(void);
 extern void initcmath(void);
 extern void initerrno(void);
 extern void initgc(void);
-#ifndef MS_WIN64
+#ifndef MS_WINI64
 extern void initimageop(void);
 #endif
 extern void initmath(void);
 extern void init_md5(void);
 extern void initnt(void);
 extern void initoperator(void);
-#ifndef MS_WIN64
+#ifndef MS_WINI64
 extern void initrgbimg(void);
 #endif
 extern void initsignal(void);
@@ -80,7 +80,7 @@
         {"array", initarray},
 	{"_ast", init_ast},
 #ifdef MS_WINDOWS
-#ifndef MS_WIN64
+#ifndef MS_WINI64
         {"audioop", initaudioop},
 #endif
 #endif
@@ -88,14 +88,14 @@
         {"cmath", initcmath},
         {"errno", initerrno},
         {"gc", initgc},
-#ifndef MS_WIN64
+#ifndef MS_WINI64
         {"imageop", initimageop},
 #endif
         {"math", initmath},
         {"_md5", init_md5},
         {"nt", initnt}, /* Use the NT os functions, not posix */
         {"operator", initoperator},
-#ifndef MS_WIN64
+#ifndef MS_WINI64
         {"rgbimg", initrgbimg},
 #endif
         {"signal", initsignal},