Dogfood `-Zno-embed-metadata` in the standard library
This PR dogfoods the [`-Zno-embed-metadata`](https://github.com/rust-lang/cargo/issues/15495) flag in the standard library. This removes the .rmeta portion out of the `libstd.so` file, thus reducing its filesize on disk. Notably, this reduces the amount of MiB that we ship to people who download the standard library.
I think that the only way to find out what this breaks is to try to run full CI, and then try to land it on nightly :)
r? @ghost
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