Rollup merge of #151317 - RalfJung:x86-soft-float, r=workingjubilee

x86 soft-float feature: mark it as forbidden rather than unstable

I am not sure why I made it "unstable" in rust-lang/rust@f755f4c; I think at the time "forbidden" did not work for some reason.

Making it "forbidden" instead has no significant effect on `-Ctarget-feature` use, it just changes the warning. It *does* have the effect that one cannot query this using `cfg(target_feature)` on nightly any more, but that seems fine to me. It only ever worked as an accidental side-effect of f755f4c anyway.

r? @workingjubilee

Author: matthiaskrgr