gcov: Add open mode parameter to gcov_do_dump()

gcc/

	* gcov-tool.cc (gcov_do_dump): Add mode parameter.
	(gcov_output_files): Open files for reading and writing.

libgcc/

	* libgcov-driver-system.c (gcov_exit_open_gcda_file): Add mode
	parameter.  Pass mode to gcov_open() calls.
	* libgcov-driver.c (dump_one_gcov):  Add mode parameter.  Pass mode to
	gcov_exit_open_gcda_file() call.
	(gcov_do_dump): Add mode parameter.  Pass mode to dump_one_gcov()
	calls.
	(__gcov_dump_one):  Open file for reading and writing.