This is a reland of 40d66d8b

The fix disambiguates duplicate symbols in the generated embedded
builtins file.

Original change's description:
> [build] Make separate snapshot for trusted variant
>
> This enables side-by-side snapshots with and without untrusted-code
> mitigations. It'll be the default in all V8 stand-alone builds
> with external startup data. Internal snapshots are not supported.
>
> The files snapshot_blob.bin and snapshot_blob_trusted.bin will be
> bundled with V8 on swarming and the correct file is loaded dependent
> on the --untrusted-code-mitigations runtime flag.
>
> Likewise we embed two snapshots for builtins.
>
> Side-by-side snapshots won't be supported in Chromium.
>
> Bug: v8:7441
> Change-Id: I2949ddfd5773649946b1c8e74751d48ad1d9c524
> Reviewed-on: https://chromium-review.googlesource.com/960004
> Commit-Queue: Michael Achenbach <machenbach@chromium.org>
> Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52028}

Bug: v8:7441
Change-Id: I626171d4e07389f0453b4d0a698e2772fd37e8c5
Reviewed-on: https://chromium-review.googlesource.com/968623
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52055}