Allocate the address map on the psymtab obstack

After this patch, the psymtab address map will now be allocated on the
psymtab obstack rather than the objfile obstack.  This also changes
the psymtab storage object to make the obstack private; this will be
used later.

gdb/ChangeLog
2019-01-10  Tom Tromey  <tom@tromey.com>

	* psymtab.h (psymtab_storage::obstack): New method.
	<m_obstack>: Rename from obstack; now private.
	* psymtab.c (psymtab_storage): Update.
	* dwarf2read.c (create_addrmap_from_index)
	(create_addrmap_from_aranges, dwarf2_build_psymtabs_hard):
	Update.