Add walltime function for the PPU

This commit adds the function
walltime() in the new file
common/ppu/walltime.h.
Add functions for a STREAM-like benchmark

This commit adds the file stream.h which
can be used to implement STREAM-like tests
for the memory bandwidth (the "-like" is
quite important here!).
Add ppu1 benchmark

This is an unoptimized
version of the benchmark.
Add ppu2 benchmark

This is a version of ppu1
with manual vectorization.
Add ppu3 benchmark

This version of the benchmark
adds loop unrolling on top of
ppu2. The code is generated via
the generate.py script such
that it is easy to vary the
unroll factor.