gcov: fix output location for JSON mode.

	PR gcov-profile/89961

gcc/ChangeLog:

	* gcov.c (make_gcov_file_name): Rewrite using std::string.
	(mangle_name): Simplify, do not used the second argument.
	(strip_extention): New function.
	(get_md5sum): Likewise.
	(get_gcov_intermediate_filename): Handle properly -p and -x
	options.
	(output_gcov_file): Use string type.
	(generate_results): Likewise.
	(md5sum_to_hex): Remove.