rename test file

because it now specifically checks for directives only used by
non-macos, non-windows x86_64

Author: folkertdev

tests/codegen/naked-fn/naked-functions.rs renamed to tests/codegen/naked-fn/x86_64-linux.rs

@@ -1,5 +1,6 @@
 //@ compile-flags: -C no-prepopulate-passes -Copt-level=0
 //@ needs-asm-support
+//@ only-linux
 //@ only-x86_64
 
 #![crate_type = "lib"]