Use -0.0 as the neutral additive float

-0.0 +  0.0 is  0.0
-0.0 + -0.0 is -0.0

Thus, the float additive-zero is actually -0.0, not its positive cousin.
This change aligns with a recent change to the impl of Sum for floats,
in rust-lang/rust@4908188 and
accordingly we also have to use the latest toolchain for our tests now.

crates/core_simd/src/simd/num/float.rs

@@ -419,7 +419,7 @@ macro_rules! impl_trait {
                     self.as_array().iter().sum()
                 } else {
                     // Safety: `self` is a float vector
-                    unsafe { core::intrinsics::simd::simd_reduce_add_ordered(self, 0.) }
+                    unsafe { core::intrinsics::simd::simd_reduce_add_ordered(self, -0.) }
                 }
             }
 

rust-toolchain.toml

@@ -1,3 +1,3 @@
 [toolchain]
-channel = "nightly-2024-06-13"
+channel = "nightly-2024-09-11"
 components = ["rustfmt", "clippy", "miri", "rust-src"]