Replace rustfmt::skip custom inner attribute with rustfmt.toml

Author: mati865

Diff for: crates/core_arch/rustfmt.toml

@@ -0,0 +1,3 @@
+ignore = [
+    "src/simd.rs",
+]

Diff for: crates/core_arch/src/simd.rs

@@ -1,6 +1,5 @@
 //! Internal `#[repr(simd)]` types
 
-#![rustfmt::skip]
 #![allow(non_camel_case_types)]
 
 macro_rules! simd_ty {