Add a TurboFan skeleton for StringAddStub.

Add a TurboFan skeleton for StringAddStub.

Currently the stub simply calls out to the runtime, this will be
improved in a later CLs. The current state at least avoids bit-rot and
later merging horror.

Fixes frame construction logic for stubs, too, and contains quite a few
tiny cleanups in stub-land.

Review URL: https://codereview.chromium.org/1150673002

Cr-Commit-Position: refs/heads/master@{#28555}