Before the "debug" flag was stored on the {CompilationEnv}. But each
background compilation task only gets the {CompilationEnv} once when
starting compilation, so by the time it picks up the "Liftoff for
debugging" compilation jobs, it might still compile them without the
debug flag being set. This leads to flakes in the "debug-step-into-wasm"
test, because we won't stop in the function prologue when stepping in
(because the function prologue does not check the "hook on function
call" flag if debug mode was not enabled).

This CL does not increase the size of a compilation unit, since both the
tier and the debug flag only need a single byte each.

As a nice side effect, this change allows us to remove the lock in
{CreateCompilationEnv}, because no modifyable flag is read any more.

R=thibaudm@chromium.org

Bug: v8:10410
Change-Id: Ic296ea0c4dd1d4dedde119f0536e87e5d301b5a1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2144116Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67115}