sim: smp: plumb igen flag down to all users

While mips has respected sim_igen_smp at configure time (which was
always empty since it defaulted smp to off), no other igen port did.
Move this to a makefile variable and plumb it through the common
IGEN_RUN variable instead so everyone gets it by default.  We also
clean up some redundant -N0 setting with multirun mips.