[Patch/C++]: Use dllonexit for static destructors on mingw32

[Benjamin Kosnik]

Yay. This will make a big difference in QoI of C++ on mingw32.

You should check out { dg-require-cxa-atexit "" } testcases, and see if
this helps. I know that libstdc++ (and I think others) have this.

testsuite/ext/mt_allocator/deallocate_local_thread-5.cc
testsuite/ext/mt_allocator/deallocate_local_thread-7.cc
testsuite/ext/mt_allocator/deallocate_local-6.cc
testsuite/ext/mt_allocator/deallocate_local-8.cc

-benjamin