alloc gas seginfo on notes obstack

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.