Address #156: Add two-arg-<= and two-arg->=
This brings us more in line with sparc and ppc. Also, x86 didn't define static functions for `two-arg-<`, `two-arg->`, `two-arg-=`, and `two-arg-/=`. Also add `generic-<=` and `generic->=` for consistency with sparc and ppc. Don't know why these aren't defined for x86, but let's add them for consistency with sparc and ppc.