base: natural default align in unmanaged singleton

I have no idea where the previous default alignment = 2 in the unsynchronized
singleton came from but as at least the Arndale IC-driver in base-hw needs an
alignment of 4 = address width, unmanaged singleton now uses sizeof(addr_t)
as default alignment.

ref #989

base/src/base/include/unmanaged_singleton.h

@@ -51,7 +51,7 @@ inline void * operator new(Genode::size_t, void * p) { return p; }
  *
  * \return  object pointer
  */
-template <typename T, int ALIGNMENT = 2, typename... ARGS>
+template <typename T, int ALIGNMENT = sizeof(Genode::addr_t), typename... ARGS>
 static inline T * unmanaged_singleton(ARGS... args)
 {
 	/*