Do not error for unknown attributes at stage0

[Manishearth]

Makes PRs like #21824 painful

[Manishearth]

(I can mentor this, btw, but please ask first, I might have finished it since this is something we need)

[Manishearth]

The relevant code is here. The custom_attribute gate should be within a #[cfg(not(stage0))] block.

[Manishearth]

This won't work. The correct fix is to use #![cfg_attr(stage0, feature(custom_attribute))] on every crate

[awlnx]

any tests needed to be ran other than compilation?

[Manishearth]

You can run make check-stage1-cfail check-stage1 rpass if you want. This is something that's really only going to be verified when a snapshot gets made.

An easy way to test this is to just add a #[foo] somewhere in the libstd, and check that it compiles at stage0 but fails at stage1

[Manishearth]

This was fixed (#23056)