closes #45085 this commit adds an `atomic_cas` target option and an unstable `#[cfg(target_has_atomic_cas)]` attribute to enable a subset of the `Atomic*` API on architectures that don't support atomic CAS natively, like MSP430 and ARMv6-M.
librustc_target
contains some very low-level details that are
specific to different compilation targets and so forth.
For more information about how rustc works, see the rustc guide.