update changelog

Author: japaric

CHANGELOG.md

@@ -19,7 +19,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Changed
 
 * Optimize the codegen of `Vec::clone`
-* `riscv32i` targets unconditionally (`no-default-features`) depends on `atomic-polyfill`
+* `riscv32i` and `riscv32imc` targets unconditionally (e.g. `build --no-default-features`) depends on `atomic-polyfill`
 
 ### Fixed