cmd/link: grow rdBuf lazily

cmd/link: grow rdBuf lazily

Counting the final buffer size usually doesn't result in the buffer growing,
so assume that it doesn't need to grow and only grow if necessary.

name       old secs    new secs    delta
LinkCmdGo   0.49 ± 4%   0.48 ± 3%  -1.31%   (p=0.000 n=95+95)

name       old MaxRSS  new MaxRSS  delta
LinkCmdGo   122k ± 4%   121k ± 5%    ~     (p=0.065 n=96+100)

Change-Id: I85e7f5688a61ef5ef2b1b7afe56507e71c5bd5b1
Reviewed-on: https://go-review.googlesource.com/21509
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Shahar Kohanim <skohanim@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>