cmd/link: more idiomatic object reader

cmd/link: more idiomatic object reader

name       old secs    new secs    delta
LinkCmdGo   0.52 ± 3%   0.52 ± 7%    ~     (p=0.325 n=93+100)

name       old MaxRSS  new MaxRSS  delta
LinkCmdGo   120k ± 1%   118k ± 4%  -1.10%   (p=0.000 n=87+96)

Change-Id: I967660b8dc6036d28eeea1b6b30f400fadd57b05
Reviewed-on: https://go-review.googlesource.com/21372
Run-TryBot: Shahar Kohanim <skohanim@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>