Only set feature(generic_nonzero) if using libcore's NonZero

Author: dtolnay

tests/ui/abi/compatibility.rs

@@ -63,8 +63,9 @@
   [csky] compile-flags: --target csky-unknown-linux-gnuabiv2
   [csky] needs-llvm-components: csky
 */
-#![feature(generic_nonzero, rustc_attrs, unsized_fn_params, transparent_unions)]
+#![feature(rustc_attrs, unsized_fn_params, transparent_unions)]
 #![cfg_attr(not(host), feature(no_core, lang_items), no_std, no_core)]
+#![cfg_attr(host, feature(generic_nonzero))]
 #![allow(unused, improper_ctypes_definitions, internal_features)]
 
 // FIXME: some targets are broken in various ways.