u128 alignment is incorrect on x86_64

[strega-nil]

In C and C++, on x86_64 Linux, alignof(__int128) is equal to 16. However, in Rust, align_of::<u128>() is equal to 8.

C++: https://gcc.godbolt.org/z/YAq1XC
Rust: https://gcc.godbolt.org/z/QvZeqK

This will cause subtle UB if you ever try to use i128s across FFI boundaries; for example:

C++: https://gcc.godbolt.org/z/PrtHlp
Rust: https://gcc.godbolt.org/z/_SdVqD

[leonardo-m]

See also: #54341

[nagisa]

Duplicate of #54341, yes.