rustc: target_features: allow for cfg-only stable target_features

[romancardenas]

This PR introduces a new stabilization level for target_features: CfgOnlyStable. The motivation is allowing the Rust compiler to expose target_features of targets so users can use cfg(target_feature = "feature") for conditional blocks depending on target features. However, CfgOnlyStable cannot be used for #[target_feature(enable = "feature")], as this is still considered unstable. Accordingly, the compiler will still raise an error if these expressions are used on stable.

This PR relates partially to #150257. As discussed, for RISC-V targets, having the "d", "e", and "f" target features exposed will allow baremetal developers to adapt the code depending on the target's properties.

r? @RalfJung

[rustbot]

r? @nikomatsakis

rustbot has assigned @nikomatsakis.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 73 candidates
• Random selection from 20 candidates

[rustbot]

RalfJung is not on the review rotation at the moment.
They may take a while to respond.