We accidentally always enabled retpolines for indirect calls in
https://crrev.com/c/1047385. This regresses performance and code size
unnecessarily if the --no-untrusted-code-mitigations flag is used.

R=titzer@chromium.org

Bug: chromium:840376, chromium:798964
Change-Id: I6bab130e33d0dafa1f547ebf7e7930a23c4eba20
Reviewed-on: https://chromium-review.googlesource.com/1057128Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53142}