extend dl-minimal malloc implementation

[DJ Delorie]

"Carlos O'Donell" <carlos@redhat.com> writes:
> (b) Eliminate all alloca by moving to malloc (requires (a)).

I will point out the obvious, for the record, that doing (b) may be
arbitrarily non-trivial.

I've looked at some of the alloca code in ld.so and the original
programmers (rightly) took advantage of the auto-cleanup, resulting in
code that might be difficult to convert.