[plt-scheme] Debugging non-scheme-heap memory leaks

The bug is fixed in SVN.

The problem was specific to the combination of x86_64, Linux, and the
JIT, because that combination triggers a use of libunwind for
generating stack traces. Disabling optimization exposed the bug by
changing ELF information so that its interpretation requires
allocation; due to the way that MzScheme calls libunwind, the allocated
memory wasn't freed. The solution was to simply enable caching in
libunwind, so that the stack-trace information isn't recomputed (and
therefore isn't re-allocated).

Thanks for the report and the very helpful detective work!