move sys fns into c-stack-cdecl and get_type_desc() into rusti

there is one test failure, stdtest/sys.rs, which inexplicably
(thus far) fails to compile because it invokes
sys::rustrt::last_os_error() instead of invoking
sys::last_os_error().  If stdtest/sys.rs is updated to invoke
the wrapper, it passes.  Still tracing the source of this error.

src/rt/rust_builtin.cpp

@@ -92,7 +92,7 @@ leak(void *thing) {
 }
 
 extern "C" CDECL intptr_t
-refcount(type_desc *t, intptr_t *v) {
+refcount(intptr_t *v) {
     // Passed-in value has refcount 1 too high
     // because it was ref'ed while making the call.
     return (*v) - 1;
@@ -109,11 +109,6 @@ unsupervise() {
     task->unsupervise();
 }
 
-extern "C" CDECL type_desc*
-get_type_desc(void *unused_task, type_desc* t) {
-    return t;
-}
-
 extern "C" CDECL void
 vec_reserve_shared(type_desc* ty, rust_vec** vp,
                    size_t n_elts) {