Add the arm architecture to the list of archs that support
Liftoff in mjsunit and so run the Liftoff tests for it.

Bug: v8:6600
Change-Id: I4896f0727f6ccc3343f5d517e100840f76dd901d
Reviewed-on: https://chromium-review.googlesource.com/c/1357040Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57976}