syslook cannot be called safely during SSA construction.

Change-Id: Ief173babd2e964fd5016578073dd3ba12e5731c5
Reviewed-on: https://go-review.googlesource.com/107815
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>