Refactor the AllocateAssemblerBuffer helper for the new Assembler API.
This is the only non-mechanical part, all other callsites that create
Assembler instances can be trivially changed to the new API. This will
be done in a separate CL.

R=mstarzinger@chromium.org

Bug: v8:8689, v8:8562
Change-Id: I6c150748eeea778d9b70f41fd66fbb1221035a1b
Reviewed-on: https://chromium-review.googlesource.com/c/1415490
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58881}