cmd/link: make Allsym a slice

cmd/link: make Allsym a slice

Looks a tiny bit faster, which is a surprise. Probably noise.
Motivation is making the LSym structure a little easier to understand.

Linking juju, best of 10:

before: real 0m4.811s user 0m5.582s
after:  real 0m4.611s user 0m5.267s

Change-Id: Idbedaf4a6e6e199036a1bbb6760e98c94ed2c282
Reviewed-on: https://go-review.googlesource.com/20142
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>