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 agree, there is a lot of SSE in many emulators. It's a shame as often, emulators whose code will remains for years have bad portability. It's also a reason why I tent to prefer shortness and brevity over _compiler compliant_ writing: Emulator code should focus on readability over performance as it make life of potential future contributors easier.

I'm not a compiler expert but if we convert this:
```c
for (j = 0; j < 4; j++)
    summand[0][j] = leftrow[0] * row[0][j];
for (j = 0; j < 4; j++)
    summand[1][j] = leftrow[1] * row[1][j];
for (j = 0; j < 4; j++)
    summand[2][j] = leftrow[2] * row[2][j];
for (j = 0; j < 4; j++)
    summand[3][j] = leftrow[3] * row[3][j];
```
To this:
```c
for (j = 0; j < 4; j++) {
    summand[0][j] = leftrow[0] * row[0][j];
    summand[1][j] = leftrow[1] * row[1][j];
    summand[2][j] = leftrow[2] * row[2][j];
    summand[3][j] = leftrow[3] * row[3][j];
}
```
Are we sure the compiler will not detect vectorizable code and provide the same result than the first case?

I'm just curious.