vterminate use of malloc-less demangler

[Benjamin Kosnik]

> The proposed patch is just using the existing callback interface from
> libiberty.  It is fairly short, adding 19 lines in one hunk and
> removing 6 in the other.  Does anybody object to it?

As one of the objectors, I will no longer stand in the way of this
patch. I believe it does solve issues with the current behavior of
__verbose_terminate_handler.

I consider better a better public interface to cxxabi.h's
__cxa_demangle function an open issue, and one not addressed by this
patch directly (since __gcclibcxx_demangle_callback is not public.)

It would be nice to have a testcase for the failure mode. If integrating
this seems difficult (it did during the last review), maybe you can
just put in a more elaborate comment in vterminate.cc by the
__gcclibcxx_demangle_callback declaration. Seem reasonable?

best,
benjamin