GNUmakefile: define HAVE_SIMD instead of separate macros for each SIMD type

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
add hand-coded AVX intrinsic implementation

The compiler still beats the pants off this thing, but it's interesting to
have.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody.cu: add nicknames for intrinsic variants

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