The Label class currently allows to be copied on all platforms except
for arm64, where it can not be copied or moved.
This allows too much though:
Copying a label even on another platform than arm64 might fail if the
label was linked already, because only one of the copies will be bound
later, and the other will fire a DCHECK error in its destructor.

This CL changes the restriction to never allow to copy construct or
assign a Label, but allow move construction and move assignment on all
platforms except arm64.
This will allow to place Labels in containers, as will be done in
Liftoff (except for arm64, where it still needs to be allocated on the
heap).

R=mstarzinger@chromium.org

Bug: v8:6600
Change-Id: Ic1234c2d233317eed6a3d537c13faed2c701fe13
Reviewed-on: https://chromium-review.googlesource.com/783190
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49570}