tidy: Accommodate rustfmt's preferred layout of stability attributes
Previously tidy would require that the `feature = "name_of_feature"` part of the stability attribute was on the same line as the `#[stable(` / `#[unstable(` opening part of the attribute, and that `)]` was on the same line as the last key-value pair.
That didn't work with rustfmt's preferred layout of long attributes, which is like:
```rust
#[unstable(
feature = "c_variadic",
reason = "the `c_variadic` feature has not been properly tested on \
all supported platforms",
issue = "44930"
)]
```
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