Bionic/libm: fast neon pow() for small x,y

Add a fast neon version of pow() suitable for relatively small
positive x and y (between 0 and 4).  Run the standard
implementation in all other cases.  Gives approximately 60%
performance improvement to AnTuTu FPU score.

Change-Id: I9234d37eaa6a815d1e619375f5b049c4ec88f557