libm: optimistic pow calling

Call optimized pow optimistically and revert to full range
implementation if we detect an out-of-range input.

Change-Id: I02657c2a254e9796e795a8fcc6ae43e24c9d37b9
libm: These optimizations are only for Krait

 * Issues on other chipsets. Resync with CAF and only use on specific
   chipsets (Krait, Sparrow).

Change-Id: I6a11f3f8e3250359259edf5f57e0d9e7068c5375