windows: export O_FILE_FLAG_* to be used in os.OpenFile on windows

These file flags are supported by os.OpenFile since CL 699415.

Closes golang/go#73676

Change-Id: Iaf846c9cb98c1458bbc30d05ad8a331ef1f332df
Reviewed-on: https://go-review.googlesource.com/c/sys/+/700975
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>