Fix internal error on library-level type extended locally

Fix internal error on library-level type extended locally

The compiler aborts on the local extension of a tagged type declared
at library level, with a progenitor given by an interface type having
a primitive that is a homograph of a primitive of the tagged type.

gcc/ada/ChangeLog:
	* gcc-interface/trans.c (maybe_make_gnu_thunk): Return false if the
	target is local and thunk and target do not have the same context.