remove rounding of g_N to multiple of CPU core count

We don't need it anymore, as it was only needed for the _threaded versions of
the tests. Now we can use arbitrary N values.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
add assertion for nbody_CPU_*_tiled: N should be a multiple of 1024

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody.cu: memset the g_host{SOA,AOS}_Force arrays before running the test

Without this, it's possible for these arrays to be filled with data from the
crosscheck run, and be untouched by any of the tested functions (e.g. if one of
the tested functions returns 0.0f due to failure, we wouldn't be able to tell
because the arrays contain a perfect match for the crosscheck).

Signed-off-by: Steven Noonan <steven@uplinklabs.net>