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

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