In particular, MIPS now has COMPAT_FREEBSD32 for n64 kernels so this
cannot be ignored for n64. On the other hand, it is unneeded for o32
MIPS kernels as the issue is only present when using 64-bit registers,
so remove the workaround from o32 kernels.
Reviewed by: jmallett
Sponsored by: DARPA / AFRL