we only expect one hit, right? otherwise something fishy is going on?

```python
compiler_rt_src_dirs = glob.glob('compiler-rt-*')
if len(compiler_rt_src_dirs) != 1:
    self.log.error("Failed to find exactly one compiler-rt source directory: %s" % compiler_rt_src_dirs)
compiler_rt_src_dir = compiler_rt_src_dirs[0]