Avoid using luau specific extension in runtime.lua

[trcrsired]

This would avoid runtime issue for luau on luajit, though there are other issues too.

[Rerumu]

Luau runtime has recently updated with further optimizations specific to it. As such if we want a more generic implementation we'd need another codegen backend rather than editing the current ones.

I'll look into that when v2 is ready, probably.