[GOLD] PowerPC .gnu.attributes support

[GOLD] PowerPC .gnu.attributes support

elfcpp/
	* powerpc.h (Tag_GNU_Power_ABI_FP): Define.
	(Tag_GNU_Power_ABI_Vector, Tag_GNU_Power_ABI_Struct_Return): Define.
gold/
	* powerpc.cc: Include attributes.h.
	(Powerpc_relobj::attributes_section_data_): New variable, with
	accessor and associated constructor and destructor support.
	(Powerpc_dynobj::attributes_section_data_): Likewise.
	(Powerpc_relobj::do_read_symbols): Stash SHT_GNU_ATTRIBUTES section
	contents in attributes_section_data_.
	(Powerpc_dynobj::do_read_symbols): Likewise.
	(Target_powerpc): Add attributes_section_data_, last_fp_, last_ld_,
	last_vec_, and last_struct_ vars.
	(Target_powerpc::merge_object_attributes): New function.
	(Target_powerpc::do_finalize_sections): Iterate over input objects
	merging attributes.  Create output attributes section.