Gameshark support

[hcs64]

It might be helpful to allow use of the Gameshark parallel port interface for writing the dumper and reading the result, in cases where a flash cartridge with SRAM aren't available.

Currently I copy the PIF ROM to 0x8030'0000 and eret back into the boot sequence. This requires that on reset the dumper gets run again, but if the cartridge is a GS it would boot back into the GS. I think it should still be possible to read back the results, though, so maybe all that is needed is a form of this that can be injected into a common game a la gsupload.