Follows https://codereview.chromium.org/2931143002/. Sufficient to link
d8 with target_os="fuchsia" in Chrome. No implementations of platform
functions yet, just stubs.

BUG=chromium:731217

Review-Url: https://codereview.chromium.org/2932053004
Cr-Commit-Position: refs/heads/master@{#45832}