riscv: promote d, e, and f target_features to CfgStableToggleUnstable Reference PR: - https://github.com/rust-lang/reference/pull/2274 This PR is a continuation of rust-lang/rust#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 rust-lang/rust#150257 r? @RalfJung