ada: Do not create null GCC thunks

ada: Do not create null GCC thunks

This prevents Gigi from creating null GCC thunks, i.e. thunks that have all
their internal parameters set to zero, replacing them with aliases.  They
can arise in degenerate cases and null thunks would trip on an assertion in
former_thunk_p when they are later optimized.

gcc/ada/

	PR ada/109817
	* gcc-interface/trans.cc (maybe_make_gnu_thunk): Create an alias
	instead of a null thunk.