enable Atomic*.{load,store} for ARMv6-M / MSP430

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.

src/liballoc/lib.rs

@@ -86,6 +86,7 @@
 #![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(cfg_target_has_atomic)]
+#![cfg_attr(not(stage0), feature(cfg_target_has_atomic_cas))]
 #![feature(coerce_unsized)]
 #![feature(collections_range)]
 #![feature(const_fn)]
@@ -162,7 +163,8 @@ mod boxed {
 #[cfg(test)]
 mod boxed_test;
 pub mod collections;
-#[cfg(target_has_atomic = "ptr")]
+#[cfg_attr(stage0, cfg(target_has_atomic = "ptr"))]
+#[cfg_attr(not(stage0), cfg(all(target_has_atomic = "ptr", target_has_atomic_cas)))]
 pub mod sync;
 pub mod rc;
 pub mod raw_vec;