syscall: add O_DIRECTORY for js

Change-Id: I2022fa27b072f9b34413a04a794aeb6d3c02166c
Reviewed-on: https://go-review.googlesource.com/c/go/+/606658


Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>