The CL also mooves the {ClearRegister} function to the
platform-independent LiftoffAssembler code.

R=clemensb@chromium.org

Bug: v8:10108
Change-Id: Ibf9f1829a525c859ad004636f678b82aa72d39ae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2129637
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Jacob Bramley <jacob.bramley@arm.com>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67126}