Windows: Add support for creating symlinks as an unprivileged user

See https://blogs.windows.com/buildingapps/2016/12/02/symlinks-windows-10/
for announcement of new flag.

This change follow the same pattern as what was done in "go":
https://github.com/golang/go/pull/24307/files#diff-b87bc12e4da2497308f9ef746086e4f0

Change-Id: If1e99fefdd3f787598e695731019c34b9bfcd1c2