`user_fpsimd_struct` on aarch64

[irgstg]

On aarch64, user_fpsimd_struct includes a __uint128_t type:
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/aarch64/sys/user.h;h=9927b3ac3c43096d85f65421c89c8bef5e0bbce9;hb=a85c93c42433aa52d4c5bc18e1dff038a82415bf#l32

There is a known issue regarding __uint128_t / __int128_t alignment to be different from u128 / i128 in rust.
I know that some of you guys faced this issue in some occasions (e.g. #2008).
In #2417 he tried few different ways to commit this struct, which fails tests.

Should we consider any other workaround, if there is any to consider?

[Gankra]

PSA: rustc actually correctly aligns u128/i128 on aarch64 linux (android) and macos and it's totally sound to use it for FFI on those platforms! So we can actually properly add these types and unblock everything that wants to use them.

rust-lang/rust#54341 (comment)