add unsafe math optimization error bounds for the non-derived atan2

[bashbaug]

In the tables describing required accuracy for math functions with "unsafe math optimizations" enabled we describe the required accuracy for both "derived" and "non-derived" implementations for most functions, but not for atan2.

This PR adds the required error bounds for atan2 as <= 8192 ulp, consistent with the required error bounds for the other non-derived implementations.

In addition, for Khronos folks, note that the required error bounds in these cases were originally added by BugZilla 12379. I believe the intent all along was to define non-derived error bounds for atan2, but in the original proposal document the table cell for atan2 wraps onto the next page and it doesn't appear that the non-derived error bounds were ever incorporated into the actual spec.