Add crate "unwind" to link with libunwind on AIX.

Author: xingxue-ibm

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

@@ -1,4 +1,4 @@
-#![feature(lang_items)]
+#![feature(lang_items, panic_unwind)]
 #![no_std]
 
 // Since `rustc` generally passes `-nodefaultlibs` to the linker,
@@ -10,6 +10,12 @@
 #[cfg(not(all(windows, target_env = "msvc")))]
 extern crate libc;
 
+// Since crate `unwind` is a dependency of crate `std`, and we are using
+// `#![no_std]`, `libunwind` is not included in the link command on AIX
+// by default. We need to include crate `unwind` manually.
+#[cfg(target_os = "aix")]
+extern crate unwind;
+
 #[panic_handler]
 pub fn begin_panic_handler(_info: &core::panic::PanicInfo<'_>) -> ! {
     loop {}