sim: hoist cgen mloop rules up to common builds

sim: hoist cgen mloop rules up to common builds

These rules don't depend on the target compiler settings, so hoist
the build logic up to the common builds for better parallelization.

We have to extend the genmloop.sh logic a bit to allow outputting
to a subdir since it always assumed cwd was the right place.

We leave the cgen maintainer rules in the subdirs for now as they
aren't normally run, and they rely on cgen logic that has not yet
been generalized.