gas .file sanity check

Currently we allow insane file numbers that cause gas to allocate up
to 4G of memory for a file array.  Trim that a little to 1G (which
still allows insane file numbers up to 33554431), and tidy function
parameter types so that we only need one file number sanity check.

	* dwarf2dbg.c (assign_file_to_slot): Take a valueT file number.
	Reduce max files array size.
	(allocate_filename_to_slot): Take a valueT file number.
	(dwarf2_directive_filename): Don't duplicate file number
	sanity check here.