Rollup merge of #55169 - raphlinus:copysign, r=joshtriplett

Add a `copysign` function to f32 and f64

This patch adds a `copysign` function to the float primitive types. It is an exceptionally useful function for writing efficient numeric code, as it often avoids branches, is auto-vectorizable, and there are efficient intrinsics for most platforms.

I think this might work as-is, as the relevant `copysign` intrinsic is already used internally for the implementation of `signum`. It's possible that an implementation might be needed in japaric/libm for portability across all platforms, in which case I'll do that also.

Part of the work towards #55107

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