Add a makefile option for wasm prototype.

Add a makefile option for wasm prototype.

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

Committed: https://crrev.com/dacb3d76a7cb651f986bb08a590e001e3bf23fbb
Cr-Commit-Position: refs/heads/master@{#30236}

[titzer, 2015-08-18 18:07:27]

lgtm

[bradn, 2015-08-18 18:08:11]

The CQ bit was checked by bradnelson@google.com

[commit-bot: I haz the power, 2015-08-18 18:08:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1293073004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1293073004/1

[commit-bot: I haz the power, 2015-08-18 18:25:22]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-18 18:25:40]

Patchset 1 (id:??) landed as
https://crrev.com/dacb3d76a7cb651f986bb08a590e001e3bf23fbb
Cr-Commit-Position: refs/heads/master@{#30236}