memcheck/tests/libstdc++.supp: rename suppression

memcheck/tests/libstdc++.supp: rename suppression

The name malloc-leaks-cxx-stl-string-classes-debug was confusing
since the suppression wasn't a leak, not part of stl, string,
classes or debug. Rename it to libstdcxx-emergency-eh-alloc-pool
to indicate it is part of the emergency exception handling memory
pool.

Note that suppression is only needed for some test cases, normally
the pool is cleaned up as part of cxx_freeres.