This PR tries to fix an incorrect usage of `if defined(EIGEN_ARCH_PPC)`
in `Eigen/Core` header.
In `Eigen/src/Core/util/Macros.h`, EIGEN_ARCH_PPC was explicitly defined
as either 0 or 1. As a result `if defined(EIGEN_ARCH_PPC)` will always be true.
This causes issues when building on non PPC platform and `MatrixProduct.h` is not
available.
This fix changes `if defined(EIGEN_ARCH_PPC)` => `if EIGEN_ARCH_PPC`.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>