Do not stream redundant stuff

Do not stream redundant stuff

as discussed on IRC this adds knob to disable stuff we stream "just for fun"
(or to make it easier to debug streamer desychnonization).

Te size of .o files in gcc subdirectory is reduced form 506MB to 492MB

gcc/

	* lto-streamer-out.c (lto_output_tree): Add streamer_debugging check.
	* lto-streamer.h (streamer_debugging): New constant
	* tree-streamer-in.c (streamer_read_tree_bitfields): Add
	streamer_debugging check.
	(streamer_get_pickled_tree): Likewise.
	* tree-streamer-out.c (pack_ts_base_value_fields): Likewise.