sim: mips: unify itable generation (a bit)

sim: mips: unify itable generation (a bit)

The m16 & multi targets generate itable once even when all the other
modules are generated multiple times.  The default igen target will
generate itable with everything else out of convenience.  This means
flags are passed which don't affect the generated itable there.

We can unify the itable generation by making sure the right -F/-M
filter variables are passed down.  Since there's already a dedicated
rule & variable in the multi build mode, generalize that and switch
the m16 & igen builds over too.

I spent a lot of time staring at this code, building for diff mips
targets, and exploring all the shell code paths.  I think this is
safe, but only time (and users) will really tell.