os: depend on Readlink only when necessary

Currently Readlink gets linked into the binary even when Executable is
not needed.

This reduces a simple "os.Stdout.Write([]byte("hello"))" by ~10KiB.

Previously the executable path was read during init time, because
deleting the executable would make "Readlink" return "(deleted)" suffix.
There's probably a slight chance that the init time reading would return
it anyways.

Updates #6853

Change-Id: Ic76190c5b64d9320ceb489cd6a553108614653d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/311790


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Tobias Klauser <tobias.klauser@gmail.com>