I’m switching to clang on Linux for my personal projects and have tried to build it from source. I’ve also built libc++ and libc++abi following documentation on libc++ website. I’ve also written a blog post (it’s actually a series of blog post about wayland, but I started with building clang and libc++ because they are my tools of choice), you can read it at http://bytepowered.blogspot.co.uk/2013/03/wayland-and-c11-programming-part-1-of-n.html
Now, I’ve almost got rid of any gcc dependency, but I can’t replace libgcc_s yet because I’m using unwind.h from gcc and it’s required for unwinding support. Is there no support inside LLVM umbrella (maybe compiler-rt) for replacing that dependency from GCC?
libunwind also provides this header. libcxxrt provides its own version with some compatibility macros to allow Itanium and ARM EH personality functions to easily share code. Between libunwind and compiler-rt, you have most of the code that you need to replace libgcc_s. This is on my todo list for FreeBSD.