Check _M_X64 (Visual Studio, Windows/x86-64) in littleendian test.

Thanks to Norfanin for the tip!
(transplanted from 43aea6bc12d72c5749741637b43095fc64abe1ae)

physfs_internal.h

@@ -970,7 +970,8 @@ char *__PHYSFS_convertToDependent(const char *prepend,
 #define PHYSFS_LIL_ENDIAN  1234
 #define PHYSFS_BIG_ENDIAN  4321
 
-#if  defined(__i386__) || defined(__ia64__) || defined(_M_IX86) || defined(_M_IA64) || \
+#if  defined(__i386__) || defined(__ia64__) || \
+     defined(_M_IX86) || defined(_M_IA64) || defined(_M_X64) \
     (defined(__alpha__) || defined(__alpha)) || \
      defined(__arm__) || defined(ARM) || \
     (defined(__mips__) && defined(__MIPSEL__)) || \