gas: output_file_close

This is mostly a tidy with the aim of being able to free
out_file_name, but it does fix a possible attempt to unlink the output
file twice (not that that matters).

	* as.h (keep_it): New global.
	* as.c (keep_it): Delete.
	(close_output_file): Delete, merged into..
	* output-file.c (output_file_close): ..here.  Delete parameter.
	* output-file.h (output_file_close): Update prototype.