`#[target_feature]` is allowed on `main`

[LeSeulArtichaut]

#[target_feature(enable = "avx2")] // no error
fn main() {}

This is a consequence of #[target_feature] being allowed on safe functions with #![feature(target_feature_11)].

cc #69098
@rustbot label T-lang T-compiler C-bug I-unsound F-target_feature_11

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-critical