Retpolines were never used for off-heap wasm code. This CL adds them.

R=titzer@chromium.org

Bug: chromium:840376, chromium:798964
Change-Id: I9f1b2150cce484f831a83663d1fb06555e7eac82
Reviewed-on: https://chromium-review.googlesource.com/1047385
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53036}