Implement int128 intrinsic

[Rinzii]

Having access to int128 intrinsic such as int128_t and uint128_t would be extremely helpful in much of the core implementation process for most functions that implement long double. The core benefit of this would be it would allow us a generic manner to handle long double without having to worry about if we've been given 80 bits or 128 bits.

The requirements would be quite stringent and it would have to be very efficient if possible. Ideally the implementation of this type would be identical in use to something like std::uint64_t.