auto merge of #6358 : crabtw/rust/mips-segstk, r=brson

I changed ```RED_ZONE_SIZE``` to ```RZ_MAC_32``` because of stack canary failure.
Here is a LLVM patch for MIPS segmented stacks.
http://people.cs.nctu.edu.tw/~jyyou/rust/mips-segstk.patch

Current test results
```
failures:
    rand::tests::test_rng_seeded_custom_seed2
    run::tests::test_forced_destroy_actually_kills
    run::tests::test_unforced_destroy_actually_kills
    time::tests::run_tests
    uv_ll::test::test_uv_ll_struct_size_addrinfo
    uv_ll::test::test_uv_ll_struct_size_uv_timer_t

segfaults:
    rt::io::option::test::test_option_writer_error
    rt::local_services::test::unwind
    rt::sched::test_swap_tasks_then
    stackwalk::test_simple
    stackwalk::test_simple_deep
```

src/libstd/ebml.rs

@@ -157,6 +157,7 @@ pub mod reader {
     }
 
     #[cfg(target_arch = "arm")]
+    #[cfg(target_arch = "mips")]
     pub fn vuint_at(data: &[u8], start: uint) -> Res {
         vuint_at_slow(data, start)
     }