stream.h: Fix warnings

This commit adds missing return
statements in functions.
ppu: Fix printf statement

Add a whitespace at the beginning.
Add spu1 code

This commit adds a first
STREAM-like benchmark that
tests the memory bandwidth
of the SPUs.
spu1: Allow specifying number of SPUs via env

If the environment variable NSPUS
is set its value is used rather
than the value of SPE_COUNT_USABLE_SPES.
Add spu2 benchmark

This commit adds a version
of the SPU benchmark that
avoids the overhead of thread
creation and tear down by
keeping the threads alive
and using mailboxes for messaging.