Add system staging dir stamp file for bazel sandwich

This stamp file is an alias to request that the system staging directory
be built. The bazel sandwich cannot know the full list of files in the
staging directory at analysis time, so it will just depend on this
instead, and then find all the files in the staging dir at execution
time.

Bug: 265127181
Test: m bazel_sandwich
Change-Id: Iee3dff4f77c9e7054b4e8297e5d96b4ef8bf1cbb