app.mk: unify codegen steps
app.mk: unify codegen steps

- No separate path for msp430; we now emit assembly instead of object code.
- Optimization is done (mostly) by llc instead of opt. This permits control of
  the optimization level during instruction selection.

uwsampa/accept,jck/accept,uwsampa/accept,jck/accept,uwsampa/accept,uwsampa/accept,jck/accept,jck/accept