[PowerPC] Fix byte ordering of ld/st with length on BE

The builtins vec_xl_len_r and vec_xst_len_r actually use the
wrong side of the vector on big endian Power9 systems. We never
spotted this before because there was no such thing as a big
endian distro that supported Power9. Now we have AIX and the
elements are in the wrong part of the vector. This just fixes
it so the elements are loaded to and stored from the right
side of the vector.