Use a bootstrap guard for modules with new target features (rust-lang#970)

The recently added modules use target features that are not yet
available in rustc compiler used for bootstrapping.

Guard them with `#[cfg(not(bootstrap))]`.

Author: tmiasko

crates/core_arch/src/x86/mod.rs

@@ -675,19 +675,27 @@ pub use self::avx512cd::*;
 mod avx512ifma;
 pub use self::avx512ifma::*;
 
+#[cfg(not(bootstrap))]
 mod avx512bitalg;
+#[cfg(not(bootstrap))]
 pub use self::avx512bitalg::*;
 
+#[cfg(not(bootstrap))]
 mod avx512gfni;
+#[cfg(not(bootstrap))]
 pub use self::avx512gfni::*;
 
 mod avx512vpopcntdq;
 pub use self::avx512vpopcntdq::*;
 
+#[cfg(not(bootstrap))]
 mod avx512vaes;
+#[cfg(not(bootstrap))]
 pub use self::avx512vaes::*;
 
+#[cfg(not(bootstrap))]
 mod avx512vpclmulqdq;
+#[cfg(not(bootstrap))]
 pub use self::avx512vpclmulqdq::*;
 
 mod bt;