x86 soft-float feature: mark it as forbidden rather than unstable
I am not sure why I made it "unstable" in f755f4cd1a; 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 f755f4cd1a anyway.
r? @workingjubilee
17ba7f246e