nbody: switch back to O3 for optimization

While x86 does fine (and in some ways better) with -Os, ARM has a lot of
performance issues (6x performance hit with reference implementation) when
biased for size optimization:

ARMv7hf with GCC 4.7.2 (-Os):
    CPU_AOS: 2663.05 ms = 1.575x10^6 interactions/s (Rel. error: 0.000000E+00)
    CPU_AOS: 2662.69 ms = 1.575x10^6 interactions/s (Rel. error: 0.000000E+00)
    CPU_AOS: 2666.26 ms = 1.573x10^6 interactions/s (Rel. error: 0.000000E+00)

ARMv7hf with GCC 4.7.2 (-O3):
    CPU_AOS: 466.88 ms = 8.984x10^6 interactions/s (Rel. error: 0.000000E+00)
    CPU_AOS: 466.87 ms = 8.984x10^6 interactions/s (Rel. error: 0.000000E+00)
    CPU_AOS: 466.89 ms = 8.983x10^6 interactions/s (Rel. error: 0.000000E+00)

Also adding -ffast-math, which has a sizable impact on the SIMD variants.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody: add -fno-strict-aliasing to prevent insane optimizations

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