Implement "unreachable" intrinsic

[RalfJung]

This program currently errors in the wrong way:

fn main() {
    unsafe { std::hint::unreachable_unchecked() }
}

says

error: unsupported operation: unimplemented (diverging) intrinsic: unreachable
  --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/hint.rs:51:5
   |
51 |     intrinsics::unreachable()
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ unimplemented (diverging) intrinsic: unreachable
   |
   = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support

The "likely not a bug" part is definitely wrong.

An implementation might land in rustc with rust-lang/rust#70324, which we'd just inherit -- so action should only be taken in Miri if that PR does not go anywhere.