ImproperCTypes: misc. adaptations

smooth things out to avoid conflicts with
rust-lang/compiler-builtins#1006
which has at time of writing not made it into rust-lang/rust's main
branch

Author: niacdoial

tests/auxiliary/minicore.rs

@@ -29,7 +29,7 @@
     asm_experimental_arch,
     unboxed_closures
 )]
-#![allow(unused, improper_c_fn_definitions, internal_features)]
+#![allow(unused, internal_features)]
 #![no_std]
 #![no_core]