cmd/link: calculate section alignment concurrently

Reduces link time for cmd/go by 1%.

Change-Id: Iad4a16db0aedc56f81ddf73ba9b632e418dc1b19
Reviewed-on: https://go-review.googlesource.com/22242Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>