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.'
I like it, too.  Unfortunately, I just checked, and it did not provide the same assembly language results.

It added dynamic shuffle instruction code due to the new `k` variable, where shuffling one element across all four single-precision floating-point elements of the 128-bit xmm was no longer as fixated when using a loop variable instead of a literal 0, 1, 2, 3 for array index.

This is unfortunate I think, since we seem to agree that this option C) at the bottom of my reply is more readable.  Maybe it will go away in a future version of the GCC compiler, and you can use it without any performance loss whatsoever.  I sometimes observe that the current version of GCC is not good at auto-vectorizing but that the upcoming release of GCC will have such volatility fixed; you can try that if you want.  I am currently compiling on 32-bit Linux with GCC 4.8.2; you might have a better version for this particular limit.

Here is the exact,  full C code I am using to debug this btw:
```float summand[4][4], leftrow[4], row[4][4];

int main(void)
{
    register unsigned int j, k;

    for (k = 0; k < 4; k++)
        for (j = 0; j < 4; j++)
            summand[k][j] = leftrow[k] * row[k][j];
    return 0;
}```

Compiled with:
`gcc -S -O3 -msse2 -o /home/no/m64/main.s /home/no/m64/main.c`
Omit the `-msse2` flag if compiling in GCC for 64-bit x86 code.
Check `main.s` for the assembly language results of the code.

It was the same with the first two methods I gave, but the third method for all its readability and flexibility seems less rigid to the compiler's ability to guarantee single-move shuffling in the SHUFPS op-code.