Auto merge of #61635 - ecstatic-morse:const-signum, r=oli-obk

Make `i*::signum` a `const fn`.

Ticks a box in #53718.

This uses a well-known branchless implementation of `signum`: `(n > 0) as i32 - (n < 0) as i32`.

Here's a [playground](https://play.rust-lang.org/?version=nightly&mode=release&edition=2018&gist=747cf191c4974bf66c9d75e509ae6e6e) comparing the two techniques. On x86 in release mode, the branchless implementation is able to replace a `mov` and `cmov` with a `sar` and `add`, so this should be a bit faster as well.

~~This is marked as a draft since I think I'll need to add `#[rustc_const_unstable]` somewhere. Perhaps the reviewer can point me in the right direction.~~

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