Improve performance of PowerPC64 ZEND_SIGNED_MULTIPLY_LONG

Improve performance of PowerPC64 ZEND_SIGNED_MULTIPLY_LONG

Detecting overflow with the XER is slow, partially because we have to
clear it before use. We can do better by using a trick where we compare
the high 64 bits of the result with the low 64 bits shifted right
63 bits.

This is 7% faster on a POWER8 running a simple testcase:

<?php
        function testcase($count = 100000000) {
                for ($i = 0; $i < $count; $i++) {
                        $x = 1;
                        $x = $x * 2;
                        $x = $x * 2;
                        $x = $x * 2;
                        $x = $x * 2;
                }
        }

        testcase();
?>