remove nbody_multiGPU_threaded

It wasn't really adding any value.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody.cu: don't print relative error if crosscheck is disabled

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
makefile: don't separate .cflags file based on BIN name

Given that we don't do the same for object files, we might get a mixed
compilation. No good.

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