Arm64 is the only platform where Labels cannot be moved, because the
assembler keeps track of pointers to Labels. On all other platforms,
there is no need to heap-allocate the Labels.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I4e98203890a8c426afa8a1db21e31f30bab892fa
Reviewed-on: https://chromium-review.googlesource.com/783210Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49572}