Rollup merge of rust-lang#65214 - Amanieu:cfg_atomic, r=alexcrichton

Split non-CAS atomic support off into target_has_atomic_load_store

This PR implements my proposed changes in rust-lang#32976 (comment) by removing `target_has_atomic = "cas"` and splitting `target_has_atomic` into two separate `cfg`s:

* `target_has_atomic = 8/16/32/64/128`: This indicates the largest width that the target can atomically CAS (which implies support for all atomic operations).
* ` target_has_atomic_load_store = 8/16/32/64/128`: This indicates the largest width that the target can support loading or storing atomically (but may not support CAS).

cc rust-lang#32976

r? @alexcrichton

src/liballoc/lib.rs

@@ -153,7 +153,7 @@ mod boxed {
 #[cfg(test)]
 mod tests;
 pub mod collections;
-#[cfg(all(target_has_atomic = "ptr", target_has_atomic = "cas"))]
+#[cfg(target_has_atomic = "ptr")]
 pub mod sync;
 pub mod rc;
 pub mod raw_vec;