[PowerPC] Provide vec_cmpne on pre-Power9 architectures in altivec.h
These overloads are listed in appendix A of the ELFv2 ABI specification without a requirement for ISA 3.0. So these need to be available on all Altivec-capable architectures. The implementation in altivec.h erroneously had them guarded for Power9 due to the availability of the VCMPNE[BHW] instructions. However these need to be implemented in terms of the VCMPEQ[BHW] instructions on older architectures.