Revert of Remove wasm compile time option and enable wasm behind a runtime flag. (patchset #54 id:1050001 of https://codereview.chromium.org/1516753007/ )

Reason for revert:
[Sheriff] Some build failures, e.g.:
https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20shared/builds/7502

Original issue's description:
> Remove wasm compile time option and enable wasm behind a runtime flag.
>
> R=titzer@chromium.org
> BUG=
>
> Committed: https://crrev.com/153f2bd47cce9d5dfa74074dda34c02731d96924
> Cr-Commit-Position: refs/heads/master@{#32955}

TBR=titzer@chromium.org,bradnelson@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1531073003

Cr-Commit-Position: refs/heads/master@{#32957}