`rustup target list` shows targets that do not exist

[brson]

On stable there aren't yet mips targets but 'target list' still shows them.

At the present this command shows all targets even when the manifest says they aren't available. We might instead hide targets that are indicated as not available. Unfortunately, the manifest is not rich enough to describe why the target isn't available, and there are two reasons: either the build failed on a non-gated target, or the target doesn't exist yet on the channel. The 'available' manifest flag was designed for the former use case (transient build failures), not the latter.

Perhaps instead of hiding unavailable toolchains we could instead say (unavailable). Not sure offhand.

[brson]

Let's just add "(unavailable)" to the text.