in particular, this makes the `c` feature for compiler-builtins an explicit opt-in, rather than silently detected by whether `llvm-project` is checked out on disk.
exposing this is necessary because the `cc` crate doesn't support cross-compiling to MSVC, and we want people to be able to run `x check --target foo` regardless of whether they have a c toolchain available.

this also uses the new option in CI, where we *do* want to optimize compiler_builtins.

the new option is off by default for the `dev` channel and on otherwise.