Updated documentation on cross building LLVM for Musl

Someone requested that I re-post this from cfe-users:

I spent some time figuring out how to get Clang and related tools to build against musl starting from a GNU libc system. The result is a GNU-less (no libgcc, libstdc++, and GNU LD, AR…) toolchain. I figured I’d share in the hopes that someone else may find it useful…

Note: Clang and friends works as expected, but I couldn’t get the sanitizers to build against Musl, so that is the only limitation that I am aware of…

Full doc here: https://gist.github.com/croepha/cdaf30638109b1a949f35aa215a7c86d

–Dave