Doc: Add the LoongArch stabilized target features

[heiher]

This PR adds the stabilized target features as part of rust-lang/rust#135015.

[ehuss]

Is there documentation that is more alive than these links? The documentation link looks like it is archived, and I don't know what else I would look at on the GitHub account. Is there a new location where the documentation lives? Is there a way to deep-link to the individual features (like wasm does)?

[heiher]

The currently available English documentation is at https://github.com/loongson/LoongArch-Documentation. If a better option becomes available in the future, I will update it. Since not all target features have fragment identifiers, they point to the nearest location.

[ehuss]

Can you say more about what's going on with the public documentation? Do you know why it is archived? Is there an actively maintained copy of the Chinese version? Are there any intentions to revive the English version, or is it moving somewhere else?

[heiher]

Of course! Based on the latest info I've received, the LoongArch-Documentation is not permanently archived and is still maintained, with updates made as needed. It's currently the only source for the referenced information in English. The Chinese version can be downloaded from: 龙芯架构参考手册卷一-r1p11.pdf. There might be a new English version released in the future, though there's no clear timeline for that yet. The potential update is more about improving the formatting rather than addressing any issues with the existing content.

[heiher]

@rustbot ready