Add the --fast-validate-asm option, which directs asm.js code
to a new parser + validator + wasm code generator,
which is then compiled using WebAssembly.

This parser takes advantage of asm.js structure to linearly parse
asm.js code, keeping a scope stack + a few additional tables to track
varibles.

BUG=v8:6090
BUG=v8:4203

R=mstarzinger@chromium.org,marja@chromium.org,vogelheim@chromium.org,kschimpf@chromium.org

Review-Url: https://codereview.chromium.org/2757693003
Cr-Commit-Position: refs/heads/master@{#44084}