rustbuild: Package librustc & co for cross-hosts
rustbuild: Package librustc & co for cross-hosts

Currently the `rust-std` package produced by rustbuild only contains the
standard library plus libtest, but the makefiles actually produce a `rust-std`
package with all known target libraries (including libsyntax, librustc, etc).
Tweak the behavior so the dependencies of the `dist-docs` step in rustbuild
depend on the compiler libraries as well (so that they're all packaged).

Closes #32984

graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust