Consistent naming of BLAS macros

I would like to extend Eigen to use MPLAPACK for mpreal. However it was time-consuming to find all the instances where I need to add code because I couldn't initially figure out the macro naming logic for the BLAS defines.

Some examples:

• GEMM_SPECIALIZATION
• EIGEN_BLAS_SYMV_SPECIALIZE / EIGEN_BLAS_SYMV_SPECIALIZATION (in the same file, but they seem to be for different purposes? I can't tell)
• EIGEN_BLAS_TRMM_L

At the very least, I think GEMM_SPECIALIZATION should be prefixed with EIGEN_BLAS_ like the rest, so I can find all defines with a grep call.