Improve performance of PowerPC64 fast_long_add_function

Detecting overflow with the XER is slow, partially because we have to
clear it before use.

PHP already has a fast way of detecting overflow in its fallback
c implementation. Overflow only occurs if the signs of the two
operands are the same and the sign of the result is different.
Furthermore, leaving it in c allows gcc to schedule the instructions
better.

This is 9% faster on a POWER8 running a simple testcase:

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

        testcase();
?>

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