Added a new "NEON read 2 data streams" benchmark

This benchmark exposes some problems or misconfiguration in Allwinner A20
(Cortex-A7) memory subsystem. If we do reads from two separate buffers at
once, then the performance drops quite significantly. It is documented that
the automatic prefetcher in Cortex-A7 can only track a single data stream:

    http://infocenter.arm.com/help/topic/com.arm.doc.ddi0464f/CHDIGCEB.html

However there is still something wrong even when using explicit PLD
prefetch. Here are some results:

 NEON read                                            :   1256.7 MB/s
 NEON read prefetched (32 bytes step)                 :   1346.4 MB/s (0.4%)
 NEON read prefetched (64 bytes step)                 :   1439.6 MB/s (0.4%)
 NEON read 2 data streams                             :    371.3 MB/s (0.3%)
 NEON read 2 data streams prefetched (32 bytes step)  :    687.5 MB/s (0.5%)
 NEON read 2 data streams prefetched (64 bytes step)  :    703.2 MB/s (0.4%)

Normally we would expect that the memory bandwidth should remain roughly
the same no matter how many data streams we are reading at once. But even
reading two data streams is enough to demonstrate big troubles.

Being able to simultaneously read from multiple data streams efficiently
is important for 2D graphics (alpha blending), colorspace conversion
(Planar YUV -> packed RGB) and many other things.