Implement copysign for libcalls

[workingjubilee]

As mentioned elsewhere¹, it's suspiciously easy to do this, so why doesn't LLVM handle it?

The answer is it might, in fact, do that², so accepting this would be a paranoid stopgap at best. Nonetheless, if it pleases you, you may!

Footnotes

1. https:/rust-lang/rust/pull/131304 ↩

2. https:/llvm/llvm-project/pull/111269 ↩

[tgross35]

abs is useful enough that you should just put it on the Float trait in src/float/mod.rs honestly, we use it everywhere (e..g.

compiler-builtins/src/float/div.rs

Lines 115 to 116 in 8a610fb

	let sign_bit = F::SIGN_MASK;
	let abs_mask = sign_bit - one;

).

This should get something in testcrate/tests, and a call in examples/intrinsics.rs to make sure either linking or a fallback works (I think you can just call the intrinsic here since the API isn't in core).

[tgross35]

Also we do have this symbol in libm (excluding the f128 version), it just isn't exposed (though we would like it to be). It seems fine to have it here until the libm situation improves somewhat, but I think this would be the first symbol that isn't part of compiler-rt.

I'll still defer to @Amanieu to confirm that though