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

Rollup merge of #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 https://github.com/rust-lang/rust/issues/32976#issuecomment-518542029 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 #32976

r? @alexcrichton