In a baremetal envionrment, you typically don’t need a libc to build builtins in my experience, but there’s another challenge which is a potentially large number of potential targets requiring the use of multilib. This area is under active development, see [RFC] Multilib, and there are still some areas for improvements such as the build system support.
If you’re interested in the baremetal target support, I’d recommend coming to the LLVM Embedded Toolchains Working Group sync up where we discuss these topics.