MIPS: Allow pre-r6 emulation on SMP MIPSr6 kernels
There's no reason for the pre-r6 instruction emulation code to be
limited to uniprocessor kernels. We already emulate atomic memory access
instructions in a way that works for SMP systems, and nothing else
should be affected. Remove the artificial limitation, allowing pre-r6
instruction emulation to be used with SMP kernels.