In the process, move the rest of the implementation PLabels into the
CodeAssembler for consistency.

Change-Id: I56872d9fc756db066f0d13d87aeb55ec04de2495
Reviewed-on: https://chromium-review.googlesource.com/c/1329687
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57474}