[bazel_sdk] Initialize @host_platforms.

Add @platforms as a submodule and initialize @host_platforms from it.

This is needed for https://fxrev.dev/1118616 to land.

Bug: 364326159
Change-Id: I275f303db162c02893391c17caa601af3bb451f5
Reviewed-on: https://fuchsia-review.googlesource.com/c/boards/emac/+/1120636
Commit-Queue: Darren Chan <chandarren@google.com>
Reviewed-by: Jiaming Li <lijiaming@google.com>