[AArch64] use rev64 to reverse strings

[AArch64] use rev64 to reverse strings

The execution time goes from 4.388s down to 0.563s on a Graviton A1
instance for the benchmark:

function reverse_strings() {
    $a = "foo";
    for ($i = 0; $i < 100000; $i++) {
        strrev($a);
        $a .= "o";
    }
}