z80pack32: Prototyping work to support 32K + 32K fixed better

Initial test work to see what is needed to handle machines that have a 32K
chunk you can page and a fixed 32K chunk (MTX512, N8VEM v2, MicroBee etc)

Buildable prototype but needs work doing on the crt0.s/binman setup to cope
with binaries containing a big hole in the middle.