bootstrap: add windows dotslash shims

Summary:
I've been using the WIP 4kb dotslash shim (from DotSlash PR 46) for a while now and it seems usable enough. They're already in use by Reindeer, too. Let's add these to make Windows bootstraps a bit easier.

X-link: facebook/buck2#970

Reviewed By: bigfootjon

Differential Revision: D75533918

Pulled By: alexlian

fbshipit-source-id: 1d7cccfe49cd1dcec009835c64f3df792997cf75

Author: thoughtpolice