Use constant size string in hash bench.php

I don't like the previous behaviour where the bytes to hash change
every time the code changes, that may make it difficult to compare
hash() performance changes over time.

Use a fixed number instead, and allow passing an override for a
different length.

Closes GH-6386.

[ci skip]

Author: divinity76

ext/hash/bench.php

@@ -3,7 +3,7 @@
 
 error_reporting(E_ALL);
 
-$data = file_get_contents(__FILE__);
+$data = str_repeat("\x00", (int) ($argv[1] ?? (2 * 1024)));
 $time = array();
 foreach (hash_algos() as $algo) {
     $time[$algo] = 0;