The wasm compiler used Pipeline::GenerateCodeForTesting to generate code
for various stubs. This change adds a dedicated entry point and moves
some common code there.

Bug: v8:8015
Change-Id: Ied628ba14c36e68826cb71d00506994184cc4763
Reviewed-on: https://chromium-review.googlesource.com/1196885
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55599}