Low-level profiler issue on OSX

@mrocklin encountered the following issue on OSX when attempting to use the low-level profiler:

```python
In [1]: from dask.distributed import Client

In [2]: client = Client(low_level_profiler=True)

In [3]: import dask.array as da

In [4]: x = da.random.random((10000, 10000), chunks=(1000, 1000))

In [5]: (x + x.T - x.mean(axis=0)).sum().compute()
libunwind: malformed __unwind_info at 0x116CF648C bad second level page # <--- here
Out[5]: 50003173.58741013
```

The error message comes from [LLVM's libunwind](https://github.com/llvm-mirror/libunwind/blob/master/src/UnwindCursor.hpp#L1699), this is possible to fail also on Linux if we attempt using LLVM.

It's not clear at the moment if this is a libunwind bug, [stacktrace](https://github.com/numba/stacktrace) or from Dask distributed.