Auto merge of rust-lang#95317 - Jules-Bertholet:round_ties_to_even, r…

…=pnkfelix,m-ou-se,scottmcm

Add `round_ties_even` to `f32` and `f64`

Tracking issue: rust-lang#96710

Redux of rust-lang#82273. See also rust-lang#55107

Adds a new method, `round_ties_even`, to `f32` and `f64`, that rounds the float to the nearest integer , rounding halfway cases to the number with an even least significant bit. Uses the `roundeven` LLVM intrinsic to do this.

Of the five IEEE 754 rounding modes, this is the only one that doesn't already have a round-to-integer function exposed by Rust (others are `round`, `floor`, `ceil`, and `trunc`).  Ties-to-even is also the rounding mode used for int-to-float and float-to-float `as` casts, as well as float arithmentic operations. So not having an explicit rounding method for it seems like an oversight.

Bikeshed: this PR currently uses `round_ties_even` for the name of the method. But maybe `round_ties_to_even` is better, or `round_even`, or `round_to_even`?

src/intrinsic/mod.rs

@@ -68,6 +68,8 @@ fn get_simple_intrinsic<'gcc, 'tcx>(cx: &CodegenCx<'gcc, 'tcx>, name: Symbol) ->
  sym::nearbyintf64 => "nearbyint",
  sym::roundf32 => "roundf",
  sym::roundf64 => "round",
+ sym::roundevenf32 => "roundevenf",
+ sym::roundevenf64 => "roundeven",
  sym::abort => "abort",
  _ => return None,
  };