Modify haxe.Int64.hx to work on php and fix other errors
Modify haxe.Int64.hx to work on php and fix other errors

- php integer overflow and ushr are incorrect (for 32-bits Int),
special functions now correct for these faults for Int64.
- both div and mod now have the sign correct when double-negative.
- special cases of div or mod by 0 or 1 now correct.

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