This CL introduces those codegen changes necessary for JIT-ing using
the WasmCodeManager.

Bug: v8:6876
Change-Id: I6b463b3e278f5e53f8dfa488f76eeaeb5231dbea
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/782261Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49554}