Refactored preparation of buffer for NBNxN table loads

The outer and inner kernels try to be hardware-agnostic, but some
architectures need a thread-local buffer for transferring computed
table indices to integer registers via memory. It is better to set up
this buffer using a helper function that is implemented in
hardware-specific files, because the compiler can help prevent some
bugs. In particular, you can no longer forget to make a function
definition on non-SSE2. This eliminates one of the special-case kernel

Change-Id: I1ebdd15afaf03ff4559365867970b88e9ce0ed5e