Add a `copysign` function to f32 and f64

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