Currently the fuzzer only tests Turbofan and Liftoff in isolation. In
order to test that both tiers use the same ABI, it should also test
calls from one tier to the other.
This CL introduces a new flag which controls which function will be
compiled by which tier, and uses that in the fuzzer.

R=ahaas@chromium.org

Bug: chromium:862931, v8:6600
Change-Id: I450b906700972cfdb496b1734faed9f8208d652f
Reviewed-on: https://chromium-review.googlesource.com/1134775
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54409}