std: implement pal::os::exit for VEXos

[tropicaaal]

This PR provides a more "proper" implementation of process exiting in VEXos programs by going through vexSystemExitRequest rather than calling intrinsics::abort, which exits using an undefined instruction trap. This matches the existing implementation of rt::abort_internal and therefore makes std::process::exit have the same behavior as returning from main on VEXos targets.

[rustbot]

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