AArch64: Optimize memset

AArch64: Optimize memset

Improve small memsets by avoiding branches and use overlapping stores.
Use DC ZVA for copies over 128 bytes.  Remove unnecessary code for ZVA sizes
other than 64 and 128.  Performance of random memset benchmark improves by 24%
on Neoverse N1.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
(cherry picked from commit cec3aef32412779e207f825db0d057ebb4628ae8)