Fcntl can't be called using syscall.Syscall as it doesn't work on AIX.
Moreover, fcntl isn't exported by syscall package.

However, it can be accessed by exporting it from runtime package
using export_aix_test.go.

Change-Id: Ib6af66d9d7eacb9ca0525ebc4cd4c92951735f1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/204059
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>