Add support for mapping emulated RAM to address 0x0

Add support for mapping emulated RAM to address 0x0

This requires a few changes, since a pointer whose value is 0x0 will be
detected as a NULL pointer. The read/write LUTs are now initialized with
0xff, and all pointers are now checked against a new INVALID_PTR macro.

Mapping the emulated RAM to the address 0x0 will allow Lightrec to
generate much better code.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>