* expose readlinkat, symlinkat

Fixes golang/go#14459

Change-Id: I8dc87401bb1025c2404e7304ecc4561edd67a452
Reviewed-on: https://go-review.googlesource.com/30890
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>