Cobalt is a V8 embedder and it has its own platform abstraction. So
to V8 the OS name is "Starboard" instead of Win/Linux/Android even
though the real OS is one of them. To select the right embedded file
writer for Cobalt, we need some customizations in
platform-embedded-file-writer-base.*.

Bug: v8:10927
Change-Id: I6dadb4690ade0b4aebec14bc87fdc6d71c03b3bb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2963804
Auto-Submit: John Xu <johnx@google.com>
Commit-Queue: John Xu <johnx@google.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75170}