Add `roundeven{,f,f16,f128}`

C23 specifies a new set of `roundeven` functions that round to the
nearest integral, with ties to even. It does not raise any floating
point exceptions.

This behavior is similar to two other functions:

1. `rint`, which rounds to the nearest integer respecting rounding mode
   and possibly raising exceptions.
2. `nearbyint`, which is identical to `rint` except it may not raise
   exceptions.

Technically `rint`, `nearbyint`, and `roundeven` all behave the same in
Rust because we assume default floating point environment. The backends
are allowed to lower to `roundeven`, however, so we should provide it in
case the fallback is needed.

Add the `roundeven` family here and convert `rint` to a function that
takes a rounding mode. This currently has no effect.

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