Add virtual destructor to GarbageCollect2Stack to silence GCC warning.
_d_allocclass -> "_d_newclass".

The define is a leftover from the times when we supported
both D1 and D2.
Correct signature of _d_newclass runtime call.

Fixes programs trying to directly access it (e.g. the
GtkD bindings). Not sure why this didn't come up earlier.
Merge pull request #526 from klickverbot/newclass-signature

Correct signature of _d_newclass runtime call.