This significantly reduces the build time when modifying wasm
files: before touching all wasm headers required 684 steps to
rebuild, now it's 216.

BUG=v8:7754,v8:7490
TBR=clemensh@chromium.org, ulan@chromium.org, tebbi@chromium.org, verwaest@chromium.org, jgruber@chromium.org

Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: I9003b5b73ac568a29688c5f97ec718c9de8aaaef
Reviewed-on: https://chromium-review.googlesource.com/1150163
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54699}