Link against -lunwind on CloudABI.

CloudABI makes use of LLVM's libunwind to do stack unwinding. It is
installed under the name libunwind.a.

Author: EdSchouten

src/libunwind/build.rs

@@ -43,5 +43,7 @@ fn main() {
         println!("cargo:rustc-link-lib=gcc_s");
     } else if target.contains("redox") {
         println!("cargo:rustc-link-lib=gcc");
+    } else if target.contains("cloudabi") {
+        println!("cargo:rustc-link-lib=unwind");
     }
 }