FFI support for `i128`/`u128`

[madsmtm]

rustc recently implemented support for the correct ABI for i128/u128, see this blog post and rust-lang/rust#54341, in particular by increasing the alignment.

We should reflect this as well in objc2 by implementing Encode/RefEncode for these types, at least once Rust removes them from the incompatible_c_types lint (tracking issue, PR).