[test][AIX] ignore extern_weak linkage test

The AIX linkage model doesn't support ELF style extern_weak semantic, so just skip this test, like other platforms that don't have it.

Author: daltenty

tests/ui/linkage-attr/propagate-generic-issue-18804/main.rs

@@ -4,6 +4,7 @@
 
 //@ ignore-emscripten no weak symbol support
 //@ ignore-apple no extern_weak linkage
+//@ ignore-aix no extern_weak linkage
 
 //@ aux-build:lib.rs