cmd/link: symbol generation optimizations

cmd/link: symbol generation optimizations

After making dwarf generation backed by LSyms there was a performance regression
of about 10%. These changes make on the fly symbol generation faster and
are meant to help mitigate that.

name       old secs    new secs    delta
LinkCmdGo   0.55 ± 9%   0.53 ± 8%  -4.42%   (p=0.000 n=100+99)

name       old MaxRSS  new MaxRSS  delta
LinkCmdGo   152k ± 6%   149k ± 3%  -1.99%    (p=0.000 n=99+97)

Change-Id: Iacca3ec924ce401aa83126bc0b10fe89bedf0ba6
Reviewed-on: https://go-review.googlesource.com/21733
Run-TryBot: Shahar Kohanim <skohanim@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>