cmd/link: patch up symbols only once per object file

cmd/link: patch up symbols only once per object file

name       old s/op   new s/op   delta
LinkCmdGo  0.57 ± 5%  0.55 ± 6%  -2.37%  (p=0.000 n=97+98)

GOGC=off:

name       old s/op   new s/op   delta
LinkCmdGo  0.48 ± 3%  0.47 ± 3%  -2.90%  (p=0.000 n=100+100)

Change-Id: I1a36dbf84914cacb79842bc0ddb1e26b4c5a5828
Reviewed-on: https://go-review.googlesource.com/20917
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>