sim: pru: move libsim.a creation to top-level

sim: pru: move libsim.a creation to top-level

The objects are still compiled in the subdir, but the creation of the
archive itself is in the top-level.  This is a required step before we
can move compilation itself up, and makes it easier to review.

The downside is that each object compile is a recursive make instead of
a single one.  On my 4 core system, it adds ~100msec to the build per
port, so it's not great, but it shouldn't be a big deal.  This will go
away of course once the top-level compiles objects.