libstdc++: optimize EH phase 2

In the ABI's two-phase EH model, first we walk the stack looking for a
handler, then we walk the stack running cleanups until we reach that
handler.  In the cleanup phase, we shouldn't redundantly check the handlers
along the way, e.g. when walking through g():

  void f() { throw 42; }
  void g() { try { f(); } catch (void *) { } }
  int main() { try { g(); } catch (int) { } }

libstdc++-v3/ChangeLog:

	* libsupc++/eh_personality.cc (PERSONALITY_FUNCTION): Don't check
	handlers in the cleanup phase.