from libstdc++ when available, similar to existing __libc_freeres().
New option --run-cxx-freeres=<yes|no> can be used to change whether
this cleanup function is called or not.
Note that __gnu_cxx::__freeres() is currently available
only in gcc 6. It is not yet decided what to do about
libstdc++ from gcc 5.
Tracked under https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69945
for libstdc++.
Fixes BZ#345307 (partially).
git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15840