Move compiler-rt build step to clang project

Firefox and geckoview builds require libclang_rt.builtins-wasm32.a to be in the clang lib directory. That step however is part of both firefox and geckoview projects builds, not clang itself. That makes it not possible to straight up use clang from tbb in a developer build, because of the missing library. It would be convenient to move this step to the clang project instead so that the toolchain from tbb can be reused in developer builds more easily.