libphobos: Give _Unwind_Exception an alignment that best resembles __attribute__((aligned))

For interoperability with C++ EH, the alignment should match, otherwise
D may not be able to intercept exceptions thrown from C++.

libphobos/ChangeLog:

	* libdruntime/gcc/unwind/generic.d (__aligned__): Define.
	(_Unwind_Exception): Align struct to __aligned__.