Modify prefetch for krait memcpy.

I originally modified the krait mainloop prefetch from cacheline * 8 to * 2.
This causes a perf degradation for copies bigger than will fit in the cache.
Fixing this back to the original * 8. I tried other multiples, but * 8 is th
sweet spot on krait.

Bug: 11221806

Change-Id: I1f75fad6440f7417e664795a6e7b5616f6a29c45