Change Cretonne's license to "Apache-2.0 WITH LLVM-exception".

This adds the "LLVM-exception" to Cretonne's existing Apache-2.0
license.

https://spdx.org/licenses/LLVM-exception.html

lib/codegen/Cargo.toml

@@ -3,7 +3,7 @@ authors = ["The Cretonne Project Developers"]
 name = "cretonne-codegen"
 version = "0.13.0"
 description = "Low-level code generator library"
-license = "Apache-2.0"
+license = "Apache-2.0 WITH LLVM-exception"
 documentation = "https://cretonne.readthedocs.io/"
 repository = "https://github.com/cretonne/cretonne"
 readme = "README.md"