Simplify z_exe_mode check

[ZornsLemma]

Thanks for taking pull request #22, as discussed there I have simplified the code further by tweaking the position of the TIMING and PRINT_SPEED checks.

[fredrikr]

I think jmp $8000 should be jmp main_loop. That makes the intention of the code clearer. This is especially important when the code modification takes place somewhere else in the code, not right next to the code being modified. Plus, if you miss calling set_z_exe_mode in ozmoo.asm, the code will still work.

Apart from that, it looks good.

[ZornsLemma]

Thanks Fredrik, I've pushed a commit to the same branch with the $8000 changed.