Fix broken C version of MulMatricesC

Me and cxd4 (Hatcat/mephostophilis) stumbled upon this while backporting some stuff to the libretro version.

The C version of MulMatricesC appears to be totally broken. I guess this went unnoticed before because it would default to the SSE2 codepath version on regular desktops.

Read the explanation by cxd4 here:

https://github.com/libretro/mupen64plus-libretro/pull/159

' 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.'