Add the missing KB multiplier. Also add a flag to set the fixed stack
size.

R=clemensb@chromium.org

Bug: v8:12191
Change-Id: I9782192d2eef1986286f726a05444a4bec49fc66
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3875902Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83065}