runtime: support nil libcall.args when calling stdcall on Windows

runtime: support nil libcall.args when calling stdcall on Windows

Having to pass a dummy pointer to the libcall.args field is a bit
annoying. This change allows nil to be passed instead.

windows/arm and windows/arm64 already support nil libcall.args.

Change-Id: I07a2bdb7d1f76b13d125397ff5177337c43536a3
Reviewed-on: https://go-review.googlesource.com/c/go/+/526016
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>