Add Fuchsia support

Adds support for the x86_64-unknown-fuchsia target, which covers the
Fuchsia operating system.

src/libstd/sys/unix/args.rs

@@ -58,7 +58,8 @@ impl DoubleEndedIterator for Args {
           target_os = "openbsd",
           target_os = "solaris",
           target_os = "emscripten",
-          target_os = "haiku"))]
+          target_os = "haiku",
+          target_os = "fuchsia"))]
 mod imp {
     use os::unix::prelude::*;
     use mem;