Use malloc for mutable addrmaps

Mutable addrmaps currently require an obstack.  This was probably done
to avoid having to call splay_tree_delete, but examination of the code
shows that all mutable obstacks have a limited lifetime -- now it's
simple to treat them as ordinary C++ objects, in some cases
stack-allocating them, and have a destructor to make the needed call.
This patch implements this change.