Use `core` via `rustc-std-workspace-core` in `library/panic*`

The three panic-related library crates need to have access to `core`,
and `compiler-builtins` needs to be in the crate graph. Rather than
specifying both dependencies, switch these crates to use
`rustc-std-workspace-core` which already does this.

This means there is now a single place that the `compiler-builtins`
dependency needs to get configured, for everything other than `alloc`
and `std`.