Fix build on go1.21

[arl]

Only tested on linux/amd64 with go1.21

First commit is just adding a go.mod, use a more recent go-sdl and runs go mod vendor Second commit fixes some runtime-internal names which have changed in the meantime (just 6 years :-) ) used by your gojit fork.

Let me know if you prefer this on rasky/gojit instead?

This codebase is full of gems, so that's my tiny contribution to it!

[arl]

Actually, I'm not sure about the jit changes. The code builds and a game rom I tried ran without any crash, but I'm not sure if the modified code is actually called... Or if I just made the linker happy