Don't mark `#[target_feature]` safe fns as unsafe in rustdoc JSON.
Fixes https://github.com/rust-lang/rust/issues/142655 by explicitly checking whether functions are safe but using `#[target_feature]`, instead of relying on the `FnHeader::is_unsafe()` method which considers such functions unsafe.
I don't believe this merits a bump of the rustdoc JSON `FORMAT_VERSION` constant, since the format is unchanged and this is just a small bugfix.
r? aDotInTheVoid
cbf7f80d5c