Fix ODR violation with `gemm_extra_cols` on PPC

When EIGEN_ALTIVEC_MMA_DYNAMIC_DISPATCH is enabled gemmMMA will instantiate gemm_extra_cols again (first in gemm). However as MatrixProductMMA.h enables #pragma GCC target("cpu=power10,htm") this leads to a different binary version of the function with the same name which is an ODR violation. This in turn leads to crashes in gemm->gemm_extra_cols when it happens to use the MMA instantiation.

Solve this by using a new name for the function similar to gemmMMA_cols vs gemm_cols.

Fixes #2608 (closed)