Until now we've been allowing unmarked asm code
through the typer. Start rejecting it.

Adding a cctest that asm modules missing an export return
fail validation.

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

Review URL: https://codereview.chromium.org/1569423002

Cr-Commit-Position: refs/heads/master@{#33199}