Enable argv support for horizon OS

See https://github.com/Meziu/rust-horizon/pull/9

library/std/src/sys/unix/args.rs

@@ -68,7 +68,8 @@ impl DoubleEndedIterator for Args {
     target_os = "l4re",
     target_os = "fuchsia",
     target_os = "redox",
-    target_os = "vxworks"
+    target_os = "vxworks",
+    target_os = "horizon"
 ))]
 mod imp {
     use super::Args;