Use an enum for file error codes.

gcc/

	* gcov-io.cc (gcov_file_error): New enum.
	(gcov_var): Use gcov_file_error enum for the error member.
	(gcov_open): Use GCOV_FILE_NO_ERROR.
	(gcov_close): Use GCOV_FILE_WRITE_ERROR.
	(gcov_write): Likewise.
	(gcov_write_unsigned): Likewise.
	(gcov_write_string): Likewise.
	(gcov_read_bytes): Set error code if EOF is reached.
	(gcov_read_counter): Use GCOV_FILE_COUNTER_OVERFLOW.