Sufficient to compile //v8 in Chrome with target_os="fuchsia". Doesn't
link yet, due to missing implementations (primarily VirtualMemory, as
well as a few utility functions).

BUG=chromium:731217
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_rel_ng

Review-Url: https://codereview.chromium.org/2931143002
Cr-Commit-Position: refs/heads/master@{#45831}