When building builtins separately as part of the runtime build, we
also want to include crt. To enable that, move the builtins CMake
build into a separate file: compiler-rt/builtins/CMakeLists.txt,
which includes both builtins and crt.
I assume that builtins and crt would be eventually moved to their own top-level directory (e.g. builtins or just compiler-rt) separate from sanitizers at which point this new CMakeLists.txt would become the root of that directory.
If I read this correctly, this would break existing setups that build builtins only, by pointing cmake to compiler-rt/lib/builtins? Is it possible to handle the transition more gracefully?