Make missed precondition-free float intrinsics safe
So, in my defence, these were both separated out from the other intrinsics in the file *and* had a different safety comment in the stable versions, so, I didn't notice them before. But, in my offence, the entire reason I did the previous PR was because I was using them for SIMD intrinsic fallbacks, and `fabs` is needed for those too, so, I don't really have an excuse.
Extra follow-up to rust-lang/rust#146683.
r? ```@RalfJung``` who reviewed the previous one
These don't appear to be used anywhere outside of the standard locations, at least.
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