dw2gencfi.c: use notes obstack

Use notes obstack for dwcfi_hash entries, and free table.  Freeing the
table makes memory checkers complain more about "definitely lost"
memory as we've moved some from the "still reachable" category.
That will be fixed with a later patch.

	* dw2gencfi.c (get_debugseg_name): Allocate on notes obstack.
	(alloc_debugseg_item): Likewise.
	(dwcfi_hash_find_or_make): Adjust failure path free.
	(cfi_finish): Delete dwfci_hash.