cmd/link: move function only lsym fields to pcln struct

cmd/link: move function only lsym fields to pcln struct

name       old secs    new secs    delta
LinkCmdGo   0.53 ± 9%   0.53 ±10%  -1.30%  (p=0.022 n=100+99)

name       old MaxRSS  new MaxRSS  delta
LinkCmdGo   151k ± 4%   142k ± 6%  -5.92%  (p=0.000 n=98+100)

Change-Id: Ic30e63a948f8e626b3396f458a0163f7234810c1
Reviewed-on: https://go-review.googlesource.com/21920
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>