Tier 2 std support must always be known

We should never have a tier 2 target without knowing its support status so I think this line in the tier 2 section is a bit wrong:

> ? indicates the standard library support is unknown or a work-in-progress.

My first inclination was just to drop the "unknown or" part. However, after thinking about it some more, I think we should just use `✓` for this. The only affected targets are UEFI and frankly there are targets with worse std support that are marked with `✓` (e.g. wasm).

I think a `✓` should mean "this supports building with std (and is checked in CI for tier 2 )". The target errata can detail the current limitations or special requirements for doing so.