use simd_neg in the impl crate::ops::Neg

on s390x I had some problems with the old implementation not combining
with other instructions well, and using the purpose-built intrinsic is
just clearer.

library/stdarch/crates/core_arch/src/powerpc/macros.rs

@@ -298,8 +298,7 @@ macro_rules! impl_neg {
         impl crate::ops::Neg for s_t_l!($s) {
             type Output = s_t_l!($s);
             fn neg(self) -> Self::Output {
-                let zero = $s::splat($zero);
-                unsafe { transmute(simd_sub(zero, transmute(self))) }
+                unsafe { simd_neg(self) }
             }
         }
     };