tidy: Accommodate rustfmt's preferred layout of stability attributes

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:

    #[unstable(
        feature = "c_variadic",
        reason = "the `c_variadic` feature has not been properly tested on \
                  all supported platforms",
        issue = "44930"
    )]