Bump exceptions submodule so that dist-boot is .gitignore'd

exceptions is a stage-0 boot library as of commit 30272412, which means that building exceptions in a GHC tree will generate a dist-boot directory. However, this directory was not specified in exceptions' .gitignore file, which causes it to dirty up the current git working directory.

Accordingly, this bumps the exceptions submodule to commit ghc/packages/exceptions@23c0b8a5, which adds dist-boot to the .gitignore file.