Unify 128bit EE reads/writes

* Initially the LQ/SQ instructions were implemented to perform two
sequential 64bit operations to emulate 128bit reads/writes. However
this won't work well for us, especially when writing to the GIF FIFO.
To mitigate this we can use the __int128 gcc extension (yay for switching
to clang once again!), which provides us with an optimized way of storing
128bit data.