Fix issue-506.rs by adding a void-returning stub in the runtime (close #2957)

Author: bblum

src/rt/rust_builtin.cpp

@@ -797,6 +797,8 @@ rust_dbg_call(dbg_callback cb, void *data) {
     return cb(data);
 }
 
+extern "C" CDECL void rust_dbg_do_nothing() { }
+
 extern "C" CDECL void
 rust_dbg_breakpoint() {
     BREAKPOINT_AWESOME;

src/rt/rustrt.def.in

@@ -170,6 +170,7 @@ rust_dbg_lock_unlock
 rust_dbg_lock_wait
 rust_dbg_lock_signal
 rust_dbg_call
+rust_dbg_do_nothing
 rust_dbg_breakpoint
 rust_osmain_sched_id
 rust_compare_and_swap_ptr

src/test/run-pass/issue-506.rs

@@ -1,4 +1,3 @@
-// xfail-test
 /*
   A reduced test case for Issue #506, provided by Rob Arnold.
 
@@ -10,10 +9,9 @@ import task;
 
 #[abi = "cdecl"]
 extern mod rustrt {
-    fn get_task_id() -> libc::intptr_t;
+    fn rust_dbg_do_nothing();
 }
 
 fn main() {
-    let f: fn() -> libc::intptr_t = rustrt::get_task_id;
-    task::spawn(unsafe { unsafe::reinterpret_cast(f) });
+    task::spawn(rustrt::rust_dbg_do_nothing);
 }