Rollup merge of #79119 - jamesmunns:patch-1, r=Mark-Simulacrum
Rollup merge of #79119 - jamesmunns:patch-1, r=Mark-Simulacrum

Clarify availability of atomic operations

This was noticed while we were updating the embedded rust book: https://github.com/rust-embedded/book/pull/273/files

These targets do natively have atomic load/stores, but do not support CAS operations.

aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust