gold: Add put_ch_reserved to 64-bit Chdr_write

The 64-bit ELF compression header has a reserved field.  It should be
cleared to avoid random bits in it.

elfcpp/

	PR gold/21857
	* elfcpp.h (Chdr_write): Add put_ch_reserved.
	(Chdr_write<64, true>::put_ch_reserved): New.
	(Chdr_write<64, false>::put_ch_reserved): Likewise.

gold/

	PR gold/21857
	* compressed_output.cc (Output_compressed_section::set_final_data_size):
	Call put_ch_reserved to clear the reserved field for 64-bit ELF.