pcw8256: don't define un-needed UAREA bits

This avoids dragging in extra code we won't be using
execve: Correct cpm emulator loading

We moved to a real off_t and lseek behaviour but that didn't get propogated
into the CP/M emulator loading logic.
socz80: Add the target

This code isn't yet complete. It's had a first pass conversion to the revised
kernel interfaces but the MMU logic has not yet been wired up to bank16 or
converted to the new format API.
socz80: Add the SocZ80 platform README
Makefile: remove reference to removed loader code