Remove rlibc from the distribution

To make progress on #18585 we're paring down the distribution to theoretically
"only libstd", and this commit makes progress on this by removing the rlibc
crate from the distribution.

The crate has now been moved into an external cargo package located in the rust
lang organization [1]. This is a breaking change due to this removal, and
existing crates depending on `rlibc` should use the Cargo crate instead.

[1]: https://github.com/rust-lang/rlibc

[breaking-change]
cc #18585