`compiler_builtins` exposes an `extern "C"` version of `libm` routines,
so add the same here. There really isn't much to test here (unless we
later add tests against C `libm` suites), but one nice benefit is this
gives us a library with unmangled names that is easy to `objdump`. In
accordance with that, also update `cb` to be a `staticlib`.
Unfortunately this also means we have to remove it from the workspace,
since Cargo doesn't allow setting `panic = "abort"` for a single crate.
We currently have a non-additive feature, "force-soft-floats", and we
will need to gain another "no-f16-f128". This makes `cfg` usage in code
somewhat confusing and redundant.
Use `build.rs` to figure out if "unstable-intrinsics" is enabled while
"force-soft-floats" is not enabled and if so, emit a cfg
`intrinsics_enabled`. This is cleaner to use and should make adding more
features easier to reason about.
Also use this as an opportunity to eliminate the build.rs from the
compiler-builtins test crate, replaced with the `[lints]` table in
Cargo.toml.
