Add OpenBLAS sbgemm.

Antonio Sánchezrequested to merge
cantonios/eigen:openblas into master

If using OpenBLAS with the BUILD_BFLOAT16=1 extension and EIGEN_USE_BLAS=1, will use OpenBLAS's sbgemm for bfloat16 * bfloat16.

Decided to use a separate macro EIGEN_USE_OPENBLAS_BFLOAT16 since the default openblas build does not include the sbgemm symbols.

Towards #2936

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