libc/arm: add cortex-a8 cpu variant

Change-Id: I30e8dd6d4b2e7889aea8f5ed21182a5941bfb489
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