remove impl Neg on s390x/powerpc vector types

[folkertdev]

For other targets we don't have such instances. We specifically decided against implementing arithmetic operators on these low-level vector types. The impl has been replaced by a vec_neg function.

Over on https://www.ibm.com/docs/en/zos/2.4.0?topic=functions-defining-vector-built-in-from-operators it is suggested to define vec_neg. We already implement the other defines on that page, so this seems like a logical extension.

On s390x vec_neg is also implemented for unsigned integer vector types https://godbolt.org/z/ddadsrn3q. vec_neg is not implemented for the bool types, see https://godbolt.org/z/TdKr7enhb).

On powerpc vec_neg is also implemented for the unsigned integer and bool vector types https://godbolt.org/z/faMbx45K3.

I plan to propose stabilization of the s390x vector types in the near future.

[rustbot]

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