x86-64: Use rep movs instructions if array copy size greater than 32 bytes
This change applies to byte and char array System.arraycopy sequence. It avoids using vector instructions for copy size between 32 bytes to 64 bytes due to an observed regression.
Signed-off-by: Annabelle Huo <Annabelle.Huo@ibm.com> (commit: 8aa9487)