gcov-tool: Add merge-stream subcommand

gcc/

	* doc/gcov-tool.texi: Document merge-stream subcommand.
	* doc/invoke.texi (fprofile-info-section): Mention merge-stream
	subcommand of gcov-tool.
	* gcov-tool.cc (gcov_profile_merge_stream): Declare.
	(print_merge_stream_usage_message): New.
	(merge_stream_usage): Likewise.
	(do_merge_stream): Likewise.
	(print_usage): Call print_merge_stream_usage_message().
	(main): Call do_merge_stream() to execute merge-stream subcommand.

libgcc/

	* libgcov-util.c (consume_stream): New.
	(get_target_profiles_for_merge): Likewise.
	(gcov_profile_merge_stream): Likewise.