Add netbsd amd64 support

src/libstd/sys/unix/process.rs

@@ -423,6 +423,7 @@ fn translate_status(status: c_int) -> ExitStatus {
               target_os = "freebsd",
               target_os = "dragonfly",
               target_os = "bitrig",
+              target_os = "netbsd",
               target_os = "openbsd"))]
     mod imp {
         pub fn WIFEXITED(status: i32) -> bool { (status & 0x7f) == 0 }