Add --target=pbc and use for build

Adds a new option `--target=pbc` to the frontend. This is the first tiny step on the way to direct bytecode generation as envisioned by whiteknight.

The corresponding NQP changes can be found in the branch `target-pbc` of the main NQP repository, and the old Parrot-side infrastructure has been removed in whiteknight's branch `eval_pmc`.

Should not be merged into `nom`: please create a new branch instead.