Auto merge of #145343 - Kobzol:dogfood-z-no-embed-metadata, r=bjorn3

Dogfood `-Zno-embed-metadata` in the standard library

This PR dogfoods the [`-Zno-embed-metadata`](rust-lang/cargo#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

Author: bors