nbody: optimize for size rather than speed

This has proven to be the best option since we're already using compiler
intrinsics, and thus we've forced the compiler to use SSE/AVX.

Here's some tests (all tests are --numbodies 64 on an cr1.8xlarge instance). No
substantial difference for clang++, but a nice performance boost for GCC.

clang++ 3.3-git, -O3 -mavx:
    CPU_SSE_threaded: 462.82 ms = 9.280x10^9 interactions/s
    CPU_SSE_threaded: 462.16 ms = 9.293x10^9 interactions/s
    CPU_SSE_threaded: 462.39 ms = 9.289x10^9 interactions/s

clang++ 3.3-git, -Os -mavx:
    CPU_SSE_threaded: 461.76 ms = 9.301x10^9 interactions/s
    CPU_SSE_threaded: 461.48 ms = 9.307x10^9 interactions/s
    CPU_SSE_threaded: 461.56 ms = 9.305x10^9 interactions/s

g++ 4.7.2, -O3 -mavx:
    CPU_SSE_threaded: 481.30 ms = 8.924x10^9 interactions/s
    CPU_SSE_threaded: 481.76 ms = 8.915x10^9 interactions/s
    CPU_SSE_threaded: 481.60 ms = 8.918x10^9 interactions/s

g++ 4.7.2, -Os -mavx:
    CPU_SSE_threaded: 472.38 ms = 9.092x10^9 interactions/s
    CPU_SSE_threaded: 472.52 ms = 9.090x10^9 interactions/s
    CPU_SSE_threaded: 473.46 ms = 9.071x10^9 interactions/s

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody: add CFOPTIMIZE and BIN variables for more command-line control

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