Fix libgcov-driver-system bootstrap failure (PR bootstrap/86057).

Fix libgcov-driver-system bootstrap failure (PR bootstrap/86057).

2018-06-07  Martin Liska  <mliska@suse.cz>

        PR bootstrap/86057
	* libgcov-driver-system.c (replace_filename_variables): Use
        memcpy instead of mempcpy.
	(allocate_filename_struct): Do not allocate filename, allocate
        prefix and set it.
	(gcov_exit_open_gcda_file): Allocate memory for gf->filename
        here and properly copy content into it.
	* libgcov-driver.c (struct gcov_filename): Remove max_length
        field, change prefix from size_t into char *.
	(compute_summary): Do not calculate longest filename.
	(gcov_do_dump): Release memory of gf.filename after each file.
	* libgcov-util.c (compute_summary): Use new signature of
        compute_summary.
	(calculate_overlap): Likewise.

From-SVN: r261260