dbghelp: Create a cpu backend to store CPU dependent code, and use it for the i386 stack implementation.

We now have fake dll files for all dlls, the icons can be loaded from there.

Edit controls don't allocate memory on the 16-bit heap anyway.

This mirrors the behavior of the 32-bit side.

dxdiagn: Fix a return value and avoid validating the input parameter for IDxDiagProviderImpl::GetContainer.

Found by Valgrind.