Add atomic load/store intrinsics to the compiler.

This pull request adds 4 atomic intrinsics to the compiler, in preparation for #5042.

* `atomic_load(src: &int) -> int` performs an atomic sequentially consistent load.
* `atomic_load_acq(src: &int) -> int` performs an atomic acquiring load.
* `atomic_store(dst: &mut int, val: int)` performs an atomic sequentially consistent store.
* `atomic_store_rel(dst: &mut int, val: int)` performs an atomic releasing store.

For more information about the whole acquire/release thing: http://llvm.org/docs/Atomics.html

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