The win64-specific unwinding info writer should not be part of the
generic EmbeddedFileWriter class. Let's hide it in the platform-specific
writer.

Bug: v8:9103
Change-Id: Ifc4f8b326f07e037b6876e0592cb70b8281edb9a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1627536
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61850}