Bug Fix: correct the bug that won't define EIGEN_HAS_FP16_C

Gengxin Xierequested to merge
xiegengxin-byted/eigen:correct-define-of-EIGEN_HAS_FP16_C into master

Reference issue

What does this implement/fix?

This patch has fixed a bug of Eigen: If we use a compiler which is not clang, EIGEN_COMP_CLANG is defined as 0, then (!defined(EIGEN_COMP_CLANG) || EIGEN_COMP_CLANG>=380)) is always false. EIGEN_HAS_FP16_C will be never defined.

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