For single-threaded, we can not tier up in the background. Hence we can
either tier up in foreground (which is pointless), or not tier up.
This CL disables Liftoff, so will compile TurboFan code right away.

R=ahaas@chromium.org

Bug: v8:10530
Change-Id: Icfdc26643ab219d617f844c2ea8e149870168dbc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2208853Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67894}