repair MulMatricesC with auto-vectorizable algorithm, remove SSE-fixed c...

...ode

The current Makefile for mupen64plus-libretro does not support the SSE1 instruction set, except of course implicitly when doing a 64-bit build on the Intel architecture.  The SSE1 algorithm could be optimized, but instead repairs were made to the ANSI C code which had broken texture elements in the backgrounds of games like _Super Mario 64_.  In addition, the new ANSI C function is written in vectorized loop style, such that GCC can very easily create the SSE instructions relevant to said loops in a manner perhaps more optimized than the original SSE-fixed code.