Rollup merge of #82374 - clehner:licenses, r=joshtriplett

Rollup merge of #82374 - clehner:licenses, r=joshtriplett

Add license metadata for std dependencies

These five crates are in the dependency tree of `std` but lack license metadata:
- `alloc`
- `core`
- `panic_abort`
- `panic_unwind`
- `unwind`

Querying the dependency tree of `std` is a useful thing to be able to do, since these crates will typically be linked into Rust binaries. Tools show the license fields missing, as seen in https://github.com/rust-lang/rust/issues/67014#issuecomment-782704534. This PR adds the license field for the five crates, based on the license of the `std` package and this repo as a whole. I also added the `repository` and `descriptions` fields, since those seem useful. For `description`, I copied text from top-level comments for the respective modules - except for `unwind` which has none.

I also note that https://github.com/rust-lang/rust/pull/73530 attempted to add license metadata for all crates in this repo, but was rejected because there was question about some of them. I hope that this smaller change, focusing only on the runtime dependencies, will be easier to review.

cc `@Mark-Simulacrum` `@Lokathor`