Record historical, no longer supported target triples in platforms crate

[lovesegfault]

I was trying to use the platform crate to parse the targets present in Rustup channel manifests, such as https://static.rust-lang.org/dist/channel-rust-1.75.0.toml but discovered not all triples are covered by this crate.

Namely, the following target triples were missing from Platform:

• asmjs-unknown-emscripten
• x86_64-sun-solaris

[tarcieri]

As it were, those are the two platforms removed in 1.76: https://github.com/rust-lang/rust/releases/tag/1.76.0#compatibility-notes

[lovesegfault]

Ah, interesting, would it be in scope for this crate to support historical platforms (i.e. this is a bug) or no (i.e. wontfix)?

[tarcieri]

It's something we could potentially do, although the current approach is to present what stable supports

[Shnatsel]

Actually we track nightly because Rust documentation does. The platforms crate is auto-generated, and some of the inputs for it are in the documentation, so we have no choice but to track whatever channel the Rust docs do.

We could keep removed platforms around, but that would require additional complexity in the generator. I'd accept a PR implementing it, but I won't write it myself since I have bigger fish to fry.