mk: mips toolchain config
rt: improve mips backend
fix mac build and comment on stack size check
Add cell#with_mut_ref for handling mutable references to the content.
auto merge of #5431 : crabtw/rust/mips-rt, r=brson

Because the PTHREAD_STACK_MIN of my system is larger than default size, I add the stack_sz check to prevent assertion failure.

Besides, libuv has to be modified because some flags are different from other targets. Instead of using hardcoded numbers, I change them to predefined symbols.

By the way, the toolchain I used is http://www.mentor.com/embedded-software/sourcery-tools/sourcery-codebench/editions/lite-edition/mips-gnu-linux

libuv patch: http://people.cs.nctu.edu.tw/~jyyou/rust/mips-uv.patch

Below is the current test result.

* core test

  stackwalk tests can cause segfault so I ignored them.

```
failures:
    io::tests::test_read_be_int_n
    io::tests::test_read_buffer_big_enough
    io::tests::test_read_f32
    io::tests::test_read_write_be
    io::tests::test_read_write_f32
    io::tests::test_read_write_le
    io::tests::test_simple
    io::tests::test_write_empty
    rand::tests::rng_seeded_custom_seed2
    unstable::uvll::test::test_uv_ll_struct_size_addrinfo
    unstable::uvll::test::test_uv_ll_struct_size_uv_timer_t

result: FAILED. 596 passed; 11 failed; 49 ignored
```

* std test:

```
failures:
    time::tests::run_tests

result: FAILED. 330 passed; 1 failed; 21 ignored
```
auto merge of #5722 : metajack/rust/cell-with-ref-mut, r=catamorphism