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

apparentlymart/golang,dsriram/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,ssrl/go,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,ssrl/go,ssrl/go,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,ssrl/go,ssrl/go,dsriram/golang,ssrl/go,ssrl/go,tv42/old-go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,gnanderson/go,tv42/old-go,apparentlymart/golang,dsriram/golang,apparentlymart/golang,gnanderson/go