add support for specifying multiple alternatives for sanity check paths

This adds support for specifying sanity check paths like:

```python
sanity_check_paths = {
    'files': ["bin/foo", ("lib/libfoo.a", "lib64/libfoo.a")],
    'dirs': [],
}
```
If `libfoo.a` is found in either `lib` or `lib64`, the sanity check would be considered OK.

Having this included would help resolve the problem with GCC lib dirs reported in https://github.com/hpcugent/easybuild-easyblocks/issues/283, and is also required for software packages like `OTF` (see https://github.com/hpcugent/easybuild-easyconfigs/pull/505), where the lib directories are different depending on the OS (e.g., `lib` on Scientific Linux, `lib64` on OpenSuSE, etc.)

We already provide flexibility like this in `make_module_req_guess` for setting `$LD_LIBRARY_PATH`, so we should also provide it for `sanity_check_paths`.