cmd/link: optimize int parsing

cmd/link: optimize int parsing

Speeds up linking cmd/go by ~1.5%:

name       old s/op   new s/op   delta
LinkCmdGo  0.58 ± 6%  0.57 ± 5%  -1.21%  (p=0.000 n=98+99)

Less noisy benchmark, with garbage collection off:

name       old s/op   new s/op   delta
LinkCmdGo  0.49 ± 2%  0.49 ± 2%  -1.79%  (p=0.000 n=98+99)

Change-Id: I0123bcb66a87cbc4d703356e4c5a4035032012ec
Reviewed-on: https://go-review.googlesource.com/20916
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>