On an Apple IIgs, the memory-mapped I/O locations are actually in
bank $e0, shadow-copied to bank $00. This adds a copy of the
relevant definitions from Cxxx-IO.sym65, with the addresses in bank
$e0 and "_GS" appended to the labels.
This is now included by default for the Apple IIgs system defintions.
(I thought about just adding them to Cxxx-IO.sym65, but then they
pollute the namespace for 8-bit systems. Stripping them out at run
time got a little complicated because the platform symbols are only
loaded once, and we'd have to reload them every time the CPU definition
changed. Further, there are a few aliases provided as constants, and
constants are allowed to be 32 bits on all systems, so those can't be
stripped. Rather than defining a new definition I figured it was
just easier to have a second file. Maintenance shouldn't be too taxing,
as definitions for 40-year-old machines don't change all that often.)
(I also thought about trying to make the address mirroring stuff work
for me here, but that would result in accesses being made to the
canonical address with an offset of +$e00000, which looks awful.)
