Adding wasm=on when invoking make will build with the wasm prototype, Ex:

make x64.debug wasm=on V=1

BUG=None
TEST=manual
R=titzer@chromium.org,ncbray@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/1293073004

Cr-Commit-Position: refs/heads/master@{#30236}