sim: mips: rename "igen" generation mode to "single"

The naming in here has grown organically and is confusing to follow.
Originally there was only one set of rules for generating code from
the igen sources, so calling it "tmp-igen" and such made sense.  But
when other multigen modes were added ("m16" & "multi") which also
used igen, it's not clear what's common igen and what's specific to
this generation  mode.  So rename the set of rules from "igen" to
"single" so it's easier to follow.