spu2: Fix comments

This commit slightly reworks
the comments in spu.c and adds
some whitespaces.
Add spu3 benchmark

This commit adds a new
version of the SPU benchmark.
The difference to spu2
is a reduced number of calls
to mfc_read_tag_status_all().
spu3: Remove unnecessary synchronization

Move the call to mfc_read_tag_status_all
into the if(unlikely(...)) branch.
spu3: Increase SIZE to 512

This slightly increases performance.
Add spu4 benchmark

spu4 uses double buffering
and achieves higher BW for
triad. For the other tests
the sustained bandwidth is
lower than for spu3 in first
measurements.