Legacy name mangling does not encode unicode characters

[tamaroning]

input:

pub fn _ユニコード(n: i32) -> i32 {
    n * n
}

actual output:

_ZN7example19_ユニコートド17heef10916c8edd029E:
        push    rbp
        ...

expected:
All non-ASCII characters are encoded as $uXXXX$:

_ZN7example36_$u30e6$$u30cb$$u30b3$$u30fc$$u30c9$17h5c5d0f1fe996ee67E:
        push    rbp
        ...