The $Cxxx I/O locations are mapped into banks $E0/E1, and are usually
configured to appear in banks $00/01 as well. Direct access to
locations in banks $E0/E1 is common in 16-bit code, but we only had
definitions for $E0.
This adds a clone of definitions for $E1, and renames the symbols
to be _E0/_E1 instead of _GS.
This can also be solved with MULTI_MASK, but that will always use
$E0 as the base address, so references to $E1/Cxxx will have a large
adjustment added ("+$10000"), which is kind of ugly.
Note we still don't have definitions for $01/Cxxx. I'll add those
if I run into them in 16-bit code. (That might be a reasonable use
of MULTI_MASK; feels less ugly somehow.)
