[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.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=47423