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.
"no method" errors on standard library types
The standard library developer can annotate methods on e.g.
`BTreeSet::push` with `#[rustc_confusables("insert")]`. When the user
mistypes `btreeset.push()`, `BTreeSet::insert` will be suggested if
there are no other candidates to suggest.
