os: use a different symbol from syscall in mkunixsignals.sh.
os: use a different symbol from syscall in mkunixsignals.sh.

Open is more likely to be present than Syscall.

R=rsc
CC=golang-dev
http://codereview.appspot.com/4645073

wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old