Add deterministic rand: d_rand()

[oboroc]

asMSX will generate same binaries across platforms and compilers for
programs that use random macro

[ghost]

I understand that the seed variable scope is the global scope so each time the function is called, the value won't be the same, hence producing a pseudo-random sequence. Am I right?

[oboroc]

Initial seed is stored in a global static variable. With built-in C library you can call srand(unsigned int) to set a new seed value. And rand() returns a value between I think 1 and 32767. I used rand() implementation from ANSI C standard:

static unsigned long int next = 1;

int rand() {
    next = next * 1103515245 + 12345;
    return (unsigned int)(next/65536) % 32768; /* RAND_MAX is 32767 */
}

void srand(unsigned int seed) {
    next = seed;
}

If you don't like ANSI C rand(), you can try adapting one of more fancy PRNGs, like Mersenne Twister.

I wanted a function that works just like rand(), but is platform and compiler independent. This way if you compile an assembly program that uses random marco, you'll get the same pseudo-random output in binary files. I didn't add srand(unsigned int) to dura.y, because asMSX never used runtime srand() to set a new PRNG seed.

Please accept this pull request, you're the only one who can do it right now. I am not sure if I can produce multiple pull requests against a repository, and I don't want to mix unrelated matters in the same pull request.