The `feature_detect` module is currently being built on all targets, but
the use of `AtomicU32` causes a problem if atomics are not available
(such as with `bpfel-unknown-none`). Gate this module behind
`target_has_atomic = "ptr"`.
The below now completes successfully:
cargo build -p compiler_builtins --target=bpfel-unknown-none -Z build-std=core
Fixes: https://github.com/rust-lang/compiler-builtins/issues/908