This ensures that there is only one entrance point from C++ to
generated code, hence only one method has to be excluded from CFI.
It also introduces type safety by only allowing the code to be called
with the right arguments.
This CL includes minor drive-by fixes in the tests, like removing
unused dummy variables.

R=mstarzinger@chromium.org

Bug: v8:7182
Change-Id: Ied9164a2497db9e7c032324c5e082094fdffc72d
Reviewed-on: https://chromium-review.googlesource.com/852213Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50426}