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

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

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#30236}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh