[PATCH v6 2/4] driver: Simplify `find_a_program` and `find_a_file`

Now that `find_a_program` and `find_a_file` have been separated, we can
make a number of simplification in both of them. Most notably, we don't
need a mode parameter, because one is always doing `R_OK`, and the other
is always doing `X_OK`.

This change also proves that some of the code I removed from
`find_a_file` in the previous commit is actually dead.

gcc/ChangeLog:

	* gcc.cc (find_a_file): Remove mode parameter, because always
	R_OK. Skip suffix logic, because suffix is always empty in that
	case.
	(read_specs): Remove mode from find_a_file call.
	(find_a_program): Suffix is unconditional, inline mode constant.
	(end_going_arg): Remove mode from find_a_file call.
	(find_file): Remove mode from find_a_file call.
	(driver::set_up_specs): Remove mode from find_a_file call.
	(include_spec_function): Remove mode from find_a_file call.

Signed-off-by: John Ericson <git@JohnEricson.me>