Fix gatomic.c compilation on Windows (MSVC)

Fix gatomic.c compilation on Windows (MSVC)

-windows.h must be included for the functions[1] to be found during
 compilation
-For Win32(x86), the InterlockedAnd, InterlockedOr and InterlockedXor
 functions need to be preceded with an underscore. This is mandated
 by the "For the x86 architecture..." clause under "Remarks"[2]
-This will close Bug 652000

[1]:http://msdn.microsoft.com/en-us/library/ms684122(v=vs.85).aspx
[2]:http://msdn.microsoft.com/en-us/library/ms683516(v=vs.85).aspx
    http://msdn.microsoft.com/en-us/library/ms683626(v=vs.85).aspx
    http://msdn.microsoft.com/en-us/library/ms684021(v=vs.85).aspx