ada: Implement support for Is_Link_Once flag on entities

ada: Implement support for Is_Link_Once flag on entities

gcc/ada/ChangeLog:

	* gcc-interface/gigi.h (create_var_decl): Add LINKONCE_FLAG boolean
	parameter.
	(create_subprog_decl): Likewise.
	* gcc-interface/decl.cc (gnat_to_gnu_entity): Adjust calls to
	create_var_decl and create_subprog_decl.
	(elaborate_expression_1): Likewise.
	* gcc-interface/trans.cc (gigi): Likewise.
	(build_raise_check): Likewise.
	(Subprogram_Body_to_gnu): Likewise.
	(create_temporary): Likewise.
	(Exception_Handler_to_gnu): Likewise.
	(Compilation_Unit_to_gnu): Likewise.
	(gnat_to_gnu): Likewise.
	(use_alias_for_thunk_p): Return false for a one-only target.
	* gcc-interface/utils.cc (maybe_pad_type): Adjust call to
	create_var_decl.
	(create_var_decl): Add LINKONCE_FLAG boolean parameter.
	(create_subprog_decl): Likewise.