[RFC] Multilib

Thanks to @petrhosek, & @MaskRay for your reviews. One patch landed, 7 to go! I know that reviewing this code is quite time consuming so I don’t want to take it for granted. I think I’ve addressed all the comments on the reviews so how do you want to proceed? Do you want me to wait for you to accept the changes or are you satisfied with the changes at a high level and happy to leave it to someone else to give a final OK for the finer details?
The LLVM Embedded Toolchains Working Group sync up is this Thursday so we can discuss then if not before.