Extract code generation (for the trampoline) from {wasm-code-manager.cc}
to a new {JumpTableAssembler}. This prepares a CL to add more logic to
the {JumpTableAssembler} to also generate jump tables for lazy
compilation and tier up.

R=mstarzinger@chromium.org

Change-Id: I383585b7e4b5a4af3ca08d07e374b44654c1a09f
Reviewed-on: https://chromium-review.googlesource.com/1046585
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53729}