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 scores

Change-Id: I97e0712daeb2740764b26a44be0caaa39c481453
(cherry picked from commit df58b162751ccad4f6a9dee1d4d8cdb5aeef39f9)
(cherry picked from commit 844736c1cd73f0118cdeb2f322c84efba0793f54)
(cherry picked from commit 8b74afbad26480891890586e811b5167f43ec28c)