gcov: Add mode to all gcov_open()

gcc/

	* gcov-io.cc (gcov_open): Always use the mode parameter.
	* gcov-io.h (gcov_open): Declare it unconditionally.

libgcc/

	* libgcov-driver-system.c (gcov_exit_open_gcda_file): Open file for
	reading and writing.
	* libgcov-util.c (read_gcda_file): Open file for reading.
	* libgcov.h (gcov_open): Delete declaration.