This makes sure that only the "asm-js.h" header is exposed to outside
the directory holding the asm.js validator. It ensures that internals
don't leak out of that component, unless they are explicitly exposed
through the defined interface.

R=clemensh@chromium.org
BUG=v8:6127

Change-Id: I7c41782254cfce102af8edf4356205cfca904e60
Reviewed-on: https://chromium-review.googlesource.com/496147Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45125}