syscall: make mkerrors.sh work with clang
syscall: make mkerrors.sh work with clang

LGTM=iant
R=golang-codereviews, minux.ma, gobot, iant
CC=golang-codereviews
https://codereview.appspot.com/67170043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang