move io.rs natives from "rust" to "c-stack-stdlib"

Author: nikomatsakis

src/lib/io.rs

@@ -1,7 +1,7 @@
 
 import os::libc;
 
-native "rust" mod rustrt {
+native "c-stack-cdecl" mod rustrt {
     fn rust_get_stdin() -> os::libc::FILE;
     fn rust_get_stdout() -> os::libc::FILE;
     fn rust_get_stderr() -> os::libc::FILE;