Add a release-notes entry for removal of Cranelift's old x86 backend.
[RFC 14]: https://github.com/bytecodealliance/rfcs/pull/14
+* Cranelift's old x86 backend has been removed, as per [RFC 12].
+ [#3309](https://github.com/bytecodealliance/wasmtime/pull/3009)
+
+[RFC 12]: https://github.com/bytecodealliance/rfcs/pull/12
+
## 0.30.0
Released 2021-09-17.
