os,syscall: pass file flags to CreateFile on Windows

os,syscall: pass file flags to CreateFile on Windows

Add support for FILE_FLAG_* constants in the flag argument of
os.OpenFile and syscall.Open on Windows.

Passing invalid flags will result in an error.

Updates #73676

Change-Id: Ie215a3dd14f0d74141533f0a07865a02a67a3846
Reviewed-on: https://go-review.googlesource.com/c/go/+/699415
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>