ENH Add support for OpenBLAS built for 64bit integers in Fortran

[jeremiedbb]

In that case, the symbols are suffixed with 64_.
Fixes #92

I tested locally with the nightly build of numpy (https://anaconda.org/scipy-wheels-nightly/numpy)

Before this PR python -m threadpoolctl -i numpy returned

[
  {
    "user_api": "blas",
    "internal_api": "openblas",
    "prefix": "libopenblas",
    "filepath": "/home/jeremie/miniconda/envs/tst-ctl/lib/python3.8/site-packages/numpy.libs/libopenblas64_p-r0-6d9684d7.3.17.so",
    "version": null,
    "num_threads": null,
    "threading_layer": "unknown",
    "architecture": null
  }
]

And with this PR

[
  {
    "user_api": "blas",
    "internal_api": "openblas",
    "prefix": "libopenblas",
    "filepath": "/home/jeremie/miniconda/envs/tst-ctl/lib/python3.8/site-packages/numpy.libs/libopenblas64_p-r0-6d9684d7.3.17.so",
    "version": "0.3.17",
    "num_threads": 4,
    "threading_layer": "pthreads",
    "architecture": "Haswell"
  }
]

(I also checked that the set_num_threads also works as expected).

If numpy/scipy eventually release with this build of OpenBLAS, the CIs with the latest versions of the packages will get this build of OpenBLAS, enabling the tests for the added code in this PR.

[ogrisel]

LGTM, thank you very much!