Personality function not required in no_std code that uses `C_unwind` FFI.

[nbdd0121]

Discovered when thinking about #97235. Exactly the same snippet of code:

extern "C-unwind" {
    fn foo();
}

fn bar() {
    unsafe { foo(); }
}

Compiling the above in -C panic=abort requires a personality function to turn unwind into abort. The current criteria for determining whether a personality function is needed just looks at the panic strategy, so rust_eh_personality isn't required, causing a linking error instead.

@rustbot label: F-c_unwind requires-nightly C-bug

[BatmanAoD]

To be clear, this should be a compile-time error, indicating a missing lang item.