Fix incorrect powf(3) result with x near 1 and |y| much larger than 1

This adjusts the check to trigger overflow/underflow to a slightly lower
value.

Before: powf(9.999995e-01, -1.342177e+08) -> inf
After: powf(9.999995e-01, -1.342177e+08) -> 1.858724e+31

MFC after: 1 week