Rollup merge of #136543 - RalfJung:round-ties-even, r=tgross35

intrinsics: unify rint, roundeven, nearbyint in a single round_ties_even intrinsic

LLVM has three intrinsics here that all do the same thing (when used in the default FP environment). There's no reason Rust needs to copy that historically-grown mess -- let's just have one intrinsic and leave it up to the LLVM backend to decide how to lower that.

Suggested by `@hanna-kruppe` in https://github.com/rust-lang/rust/issues/136459; Cc `@tgross35`

try-job: test-various

compiler/rustc_codegen_gcc/src/intrinsic/mod.rs

@@ -84,14 +84,11 @@ fn get_simple_intrinsic<'gcc, 'tcx>(
         sym::ceilf64 => "ceil",
         sym::truncf32 => "truncf",
         sym::truncf64 => "trunc",
-        sym::rintf32 => "rintf",
-        sym::rintf64 => "rint",
-        sym::nearbyintf32 => "nearbyintf",
-        sym::nearbyintf64 => "nearbyint",
+        // We match the LLVM backend and lower this to `rint`.
+        sym::round_ties_even_f32 => "rintf",
+        sym::round_ties_even_f64 => "rint",
         sym::roundf32 => "roundf",
         sym::roundf64 => "round",
-        sym::roundevenf32 => "roundevenf",
-        sym::roundevenf64 => "roundeven",
         sym::abort => "abort",
         _ => return None,
     };