cmd/link: use encbuf when writing integers

cmd/link: use encbuf when writing integers

name       old s/op   new s/op   delta
LinkCmdGo  0.59 ± 6%  0.58 ± 5%  -1.61%  (p=0.000 n=99+99)

GOGC=off:
name       old s/op   new s/op   delta
LinkCmdGo  0.50 ± 3%  0.49 ± 3%  -1.28%  (p=0.000 n=98+99)

Change-Id: I737ae056214999441a210c69ec0cf4febc39a715
Reviewed-on: https://go-review.googlesource.com/20914
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>