rustc: Blanket whitelist #[target_feature]

This commit transitions the `target_feature` attribute from `Normal` to
`Whitelisted`. Discovered in #50095 the fact of whether this attribute is used
or not is dependent on typechecking running and executing `check_name`, but
incremental compilation doesn't currently account for this, meaning that the
attribute ends up being flagged as unused when it shouldn't be.

I was a little too ambitious it seems hoping that `Normal` could be used, so
instead this transitions to `Whitelisted` to be the same as other codegen
attributes like `#[inline]`

Closes #50095

src/libsyntax/feature_gate.rs

@@ -921,7 +921,7 @@ pub const BUILTIN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeG
                                  "the `#[naked]` attribute \
                                   is an experimental feature",
                                  cfg_fn!(naked_functions))),
-    ("target_feature", Normal, Ungated),
+    ("target_feature", Whitelisted, Ungated),
     ("export_name", Whitelisted, Ungated),
     ("inline", Whitelisted, Ungated),
     ("link", Whitelisted, Ungated),