riscv: promote d, e, and f target_features to CfgStableToggleUnstable

[romancardenas]

This PR is a continuation of #155962

It uses the new CfgStableToggleUnstable stability level for the "d", "e", and "f" target_features of RISC-V. This way, it will be possible to add conditional code blocks depending on whether the target architecture has FPU, for instance.

The PR is related to #150257

r? @RalfJung

[rustbot]

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[RalfJung]

Looks good on the test side, thanks. :)
Cc @Amanieu

Let's nominate this for the @rust-lang/lang. This is about stabilizing the use of the "d", "e", and "f" RISC-V target feature for use in cfg only. IOW, this lets code check whether the features are available, but it doesn't let code enable or disable the features. That should therefore be entirely harmless ABI-wise.

[joshtriplett]

Blocked waiting on feedback from @Amanieu.

[Amanieu]

I don't see any harm in stabilizing those. The real issues comes from selectively enabling these on a per-function basis, which this doesn't do.

[traviscross]

Thanks @Amanieu.

@rfcbot fcp merge lang

[rust-rfcbot]

Team member @traviscross has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
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