Support O_SYNC flag for os.Open on windows

The current implementation of `os.Open` function does not use the O_SYNC
flag. This means that even if user has set the O_SYNC flag, the
`os.Open` call will ignore the `SYNC` flag. This commit fixes the issue
by adding the `FILE_FLAG_WRITE_THROUGH` which is the equivalent of
`O_SYNC` flag on linux.

Fixes golang#35358

src/syscall/syscall_windows.go

@@ -336,7 +336,7 @@ func Open(path string, mode int, perm uint32) (fd Handle, err error) {
 	default:
 		createmode = OPEN_EXISTING
 	}
-	var attrs uint32 = FILE_ATTRIBUTE_NORMAL
+	var attrsAndFlags uint32 = FILE_ATTRIBUTE_NORMAL
 	if perm&S_IWRITE == 0 {
 		attrs = FILE_ATTRIBUTE_READONLY
 		if createmode == CREATE_ALWAYS {
@@ -360,6 +360,11 @@ func Open(path string, mode int, perm uint32) (fd Handle, err error) {
 			}
 		}
 	}
+
+	if O_SYNC {
+		attrsAndFlags != FILE_FLAG_WRITE_THROUGH
+	}
+
 	h, e := CreateFile(pathp, access, sharemode, sa, createmode, attrs, 0)
 	return h, e
 }

src/syscall/types_windows.go

@@ -114,6 +114,7 @@ const (
 	FILE_FLAG_OPEN_REPARSE_POINT = 0x00200000
 	FILE_FLAG_BACKUP_SEMANTICS   = 0x02000000
 	FILE_FLAG_OVERLAPPED         = 0x40000000
+	FILE_FLAG_WRITE_THROUGH      = 0x80000000
 
 	HANDLE_FLAG_INHERIT    = 0x00000001
 	STARTF_USESTDHANDLES   = 0x00000100