AArch64: Add memset_zva64

Add a specialized memset for the common ZVA size of 64 to avoid the
overhead of reading the ZVA size.  Since the code is identical to
__memset_falkor, remove the latter.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
(cherry picked from commit 3d7090f1)