gas -Z flag

-Z is supposed to "generate object file even after errors".  It
doesn't do so at the moment, unless you count a zero length object
file as useful output.  Fixed for most targets as follows.
I suspect avr and pru need target patches.

	* output-file.c (output_file_close): Do a normal close when
	flag_always_generate_output.
	* write.c (write_object_file): Don't stop output when
	flag_always_generate_output.