kernel/vm_manager: Handle stack/TLS IO region placement better

Handles the placement of the stack a little nicer compared to the
previous code, which was off in a few ways. e.g.

The stack (new map) region, shouldn't be the width of the entire address
space if the size of the region calculation ends up being zero. It
should be placed at the same location as the TLS IO region and also have
the same size.

In the event the TLS IO region contains a size of zero, we should also
be doing the same thing. This fixes our memory layout a little bit and
also resolves some cases where assertions can trigger due to the memory
layout being incorrect.