powerpc: Replace lxvd2x/stxvd2x with lvx/stvx in P7's memcpy/memmove
POWER9 DD2.1 and earlier has an issue where some cache inhibited
vector load traps to the kernel, causing a performance degradation. To
handle this in memcpy and memmove, lvx/stvx is used for aligned
addresses instead of lxvd2x/stxvd2x.