addcarryx: use pointers of the right type

[RalfJung]

I don't know why these functions use u8 in their signature, but surely they must actually be writing a u32/u64?

I also synced the doc comments between addcarry and addcarryx.

[rustbot]

r? @Amanieu

rustbot has assigned @Amanieu.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[Amanieu]

Presumably this was relevant when LLVM used typed pointers, but this isn't the case now that it has switched to opaque pointers.