Fix the "used_with_archive" test on Fuchsia

This change adds Fuchsia OS as a target to the cfg_attr in the
pre_main_constructor external declaration. This allows the
"tests/ui/attributes/used_with_archive.rs" to pass against Fuchsia.

tests/ui/attributes/auxiliary/used_pre_main_constructor.rs

@@ -19,6 +19,7 @@
         target_os = "netbsd",
         target_os = "nto",
         target_os = "openbsd",
+        target_os = "fuchsia",
     ),
     link_section = ".init_array"
 )]