Add rint and rintf

[Jules-Bertholet]

Necessary to support rust-lang/rust#95317

[Jules-Bertholet]

Actually, I think it might make more sense to just use rint/rintf...

[Amanieu]

The differences between rint and roundeven are:

• rint uses the rounding mode from the FP environment (which defaults to round-to-nearest) while roundeven always rounds to nearest even.
• rint raises FE_INEXACT if rounding occurs, roundeven doesn't.

As you said though, Rust already assumes the default rounding mode, so it doesn't matter too much. I think roundeven is the "more correct" choice to use but it's not well supported so it's OK to just use rint for now. Ideally we should support both in libm.

[Jules-Bertholet]

This change needs to make its way to a stable release before rust-lang/rust#95317 can be unblocked.

[Amanieu]

I published a new version. However you will need to update compiler-builtins which includes libm as a submodule. You'll need to add a re-export here: https://github.com/rust-lang/compiler-builtins/blob/master/src/math.rs