Only use feature panic_handler for AIX.

Author: xingxue-ibm

tests/ui/extern-flag/auxiliary/panic_handler.rs

@@ -1,5 +1,6 @@
-#![feature(lang_items, panic_unwind)]
+#![feature(lang_items)]
 #![no_std]
+#![cfg_attr(target_os = "aix", feature(panic_unwind))]
 
 // Since `rustc` generally passes `-nodefaultlibs` to the linker,
 // Rust programs link necessary system libraries via `#[link()]`