make DEFAULT_HANDLER an undefined symbol

link.x

@@ -3,10 +3,14 @@ INCLUDE memory.x
 /* With multiple codegen units the rlib produced for this crate has several object files in it. */
 /* Because the linker is Smart it may not look into all the object files and not pick up the */
 /* .vector_table.exceptions section. But we want it to! To workaround the problem we create an */
-/* undefined reference to the EXCEPTIONS symbol (located in .vector_table.exceptions); this way the */
-/* linker will look at all the object of the rlib and pick up our EXCEPTIONS symbol */
+/* undefined reference to the EXCEPTIONS symbol (located in .vector_table.exceptions); this way
+/* the linker will look at all the object of the rlib and pick up our EXCEPTIONS symbol */
 EXTERN(EXCEPTIONS);
 
+/* Without this if the top (binary) crate defines the DEFAULT_HANDLER symbol (e.g. via */
+/* default_handler!) then it will be ignored / dropped */
+EXTERN(DEFAULT_HANDLER);
+
 /* Create an undefined reference to the INTERRUPTS symbol. This is required to
    force the linker to *not* drop the INTERRUPTS symbol if it comes from an
    object file that's passed to the linker *before* this crate */