Disable f16c scalar conversions for MSVC.

Antonio Sánchezrequested to merge
cantonios/eigen:msvc into master

MSVC seems to be lacking the scalar conversion instructions. It has the vector ones.

Bug reported over at TensorFlow: https://github.com/tensorflow/tensorflow/issues/54397

Similiar issue found in an intel repo: Similar issue found in an intel repo: https://github.com/intel/tinycbor/pull/193

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