Add __int128 support

[belous-dp]

Description
Add support for __int128 and unsigned __int128.

Example
#include <limits.h>

unsigned __int128 int128_mult(unsigned long long a, unsigned long long b) {
    unsigned __int128 mult = a;
    mult *= b;
    if (mult > (unsigned __int128) LLONG_MAX) {
        return ~((unsigned __int128)0);
    }
    return mult;
}

Important
After adding support for this feature, do 3 things:

1. Enable int128_mult syntax test.
2. Move tests for int128 from c-example-unsupported to c-example
3. Mark int128 as supported type on c-syntax page.