ada: Remove strange elaboration code generated for Cluster type in System.Pack_NN

ada: Remove strange elaboration code generated for Cluster type in System.Pack_NN

Initialization procedures are turned into functions under the hood and, even
when they are null (empty), the compiler may generate a convoluted sequence
of instructions that return uninitialized data and, therefore, is useless.

gcc/ada/ChangeLog:

	* gcc-interface/trans.cc (Subprogram_Body_to_gnu): Do not generate
	a block-copy out for a null initialization procedure when the _Init
	parameter is not passed in.