[patch] libstdc++/69945 Add __gnu_cxx::__freeres hook

[Jonathan Wakely]

This adds a new function to libsupc++ which will free the memory still
in use by the pool used for allocating exceptions when malloc fails.

This is similar to glibc's __libc_freeres, which valgrind (and other
tools?) use to tell glibc to deallocate everything before exiting.

I initially called it __gnu_cxx::__free_eh_pool() but I figured we
might have other memory in use at some later date, and we wouldn't
want valgrind to have to start calling a second function, nor make a
function called __free_eh_pool() actually free other things.

I intentionally *didn't* lock the pool's mutex before freeing it,
because this should never be called in a context where multiple
threads are still active and accessing the pool.

Thoughts?

Is the new test a good idea, or will exposing it like that in the
testsuite just give users the idea that they can/should be doing the
same themselves?


[ Aside: should the actual pool be created with placement-new to
[ ensure it doesn't ever get destroyed?
[
[   alignas(pool) unsigned char poolbuf[sizeof(pool)];
[   pool& emergency_pool = *::new(poolbuf) pool;



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