[PowerPC] Implement partial vector ld/st builtins for XL compatibility

XL provides functions __vec_ldrmb/__vec_strmb for loading/storing a
sequence of 1 to 16 bytes in big endian order, right justified in the
vector register (regardless of target endianness).
This is equivalent to vec_xl_len_r/vec_xst_len_r which are only
available on Power9.

This patch simply uses the Power9 functions when compiled for Power9,
but provides a more general implementation for Power8.

Differential revision: https://reviews.llvm.org/D106757