alloc gas seginfo on notes obstack

Lots of memory used in gas should go on this obstack.  The patch also
frees all the gas obstacks on exit, which isn't a completely trivial
task.

	* subsegs.c (alloc_seginfo): New function.
	(subseg_change, subseg_get): Use it.
	(subsegs_end): New function.
	* as.h (subsegs_end): Declare.
	* output-file.c: Include subsegs.h
	(stash_frchain_obs): New function.
	(output_file_close): Save obstacks attached to output bfd before
	closing.  Call subsegs_end with the array of obstacks.