Weak aliases should only point to defined symbols.

One possible solution for
https://github.com/japaric/cortex-m-rtfm/issues/39

Author: pftbest

src/generate.rs

@@ -232,10 +232,13 @@ pub fn interrupt(
         names.push(name_uc);
     }
 
-    let aliases = names.iter()
+    let aliases = "
+DH_TRAMPOLINE:
+    jmp DEFAULT_HANDLER
+".to_string() + &names.iter()
             .map(|n| format!("
 .weak {0}
-{0} = DEFAULT_HANDLER", n))
+{0} = DH_TRAMPOLINE", n))
             .collect::<Vec<_>>()
             .concat();