Support miri-specific entry point / start function for no-std binaries

[RalfJung]

For no-std binaries, Miri currently relies on the start feature, but that feature doesn't really make a lot of sense and should likely be removed. It also causes confusion for Miri users.

So we should instead say that in a no-std binary, we Miri will start the program by jumping to a function with link_name miri_start, and then if someone has a no-std binary they want to run in Miri, they can add something like

#[cfg(miri)]
#[no_mangle]
fn miri_start() {
  // Call the actual start function that your project implements, based on your target's conventions.
}