Make it possible to switch on simd.js support when combined with asm.js
in the asm->wasm path.

BUG= https://code.google.com/p/v8/issues/detail?id=4203
TEST=test-asm-validator
R=gdeepti@chromium.org,titzer@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/1643333002

Cr-Commit-Position: refs/heads/master@{#33644}