sys: add MIPS R6 support

[chenx97]

Currently, R6 targets are almost identical to their R2/R5 counterparts. So we add target_arch and bump libc.

[SteveLauC]

Thanks for doing this! I think I will give it a check after dealing with our CI migration

[chenx97]

I'll migrate my log to towncrier soon

[SteveLauC]

Upstream PRs:

• libc: add mips32r6 and mips64r6 target_arch detection
• Rust: add mips64r6 and mips32r6 as target_arch values

Targets docs: mipsisa*r6*-unknown-linux-gnu*

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Changes LGTM, but in case something could go wrong, I would like to have @asomers give this a double check

[asomers]

This would be a good candidate for cfg_aliases, which we have discussed for other purposes.
https://docs.rs/cfg_aliases/latest/cfg_aliases/

[SteveLauC]

If we are gonna replace most big-chunk, common cfg!s with some aliases, then it might be better to do it in another, dedicated PR, where we can replace them all and get them well documented in CONTRIBUTING.md

[SteveLauC]

#2188 issue filed, Let's merge this PR as-is

[asomers]

@chenx97 you'll need to rebase your PR to fix CI.

[SteveLauC]

There seems to be something wrong with our merge queue, the GHA checks simply don't run, then it reaches the timeout and gets removed from the queue

Update: seems that this is what we missed https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue#triggering-merge-group-checks-with-github-actions