AArch64: Improve backwards memmove performance

On some microarchitectures performance of the backwards memmove improves if
the stores use STR with decreasing addresses.  So change the memmove loop
in memcpy_advsimd.S to use 2x STR rather than STP.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>