cfg(bootstrap) should not be allowed in `library/`

[RalfJung]

#141925 removed bootstrap from the allowed-cfg list for the library crates, but ./x still passes in extra flags that allow cfg(bootstrap). This has already led to new cfg(bootstrap) slipping in, and it has led to issue in tooling that builds library/ crates outside of ./x (such as https://github.com/rust-lang/miri-test-libstd/).

Cc @Urgau @cuviper @onur-ozkan @workingjubilee

[Urgau]

Opened #142160, should fix the erroneous --check-cfg for bootstrap in library/, and by extension disallow it's usage.

[workingjubilee]

Thanks!