Not the normal simple update-the-hash SDK update. Changes:

- Use our own libunwind (as it's been removed from the Fuchsia SDK);

- Use the toolchain hash as a version #define instead of the .stamp in
  sysroot (which has been removed). The Fuchsia team builds with -MD to
  cause rebuilds after ABI changes, Chromium builds with -MMD. The
  global define ensures we rebuild on SDK updates;

- Update clang version to '6.0.0' to get correct builtins lib;

- Update docs for pulling SDK from prebuilt package, rather than doing
  local builds.

- Don't include libc++ and libunwind .so into the runner disk image, as
  we link to them statically now.

Bug: 707030, 750392
Change-Id: I6702ce208e23288107db00d80f4bed7f875820ae
Reviewed-on: https://chromium-review.googlesource.com/594270


Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491035}