Add needs-unwind to codegen test

When compiled with -C panic=abort we'd generate an extra
panic_cannot_unwind shim in the variant calling C-unwind.

tests/codegen/maybeuninit-rvo.rs

@@ -1,4 +1,5 @@
 //@ compile-flags: -O
+//@ needs-unwind
 //@ min-llvm-version: 18
 #![feature(c_unwind)]
 #![crate_type = "lib"]