random32: add test cases for taus113 implementation

      We generated a battery of 100 test cases from GSL taus113 implemention
and compare the results from a particular seed and a particular
iteration with our implementation in the kernel. We have verified on
32 and 64 bit machines that our taus113 kernel implementation gives
same results as GSL taus113 implementation:

  [    0.147370] prandom: seed boundary self test passed
  [    0.148078] prandom: 100 self tests passed

This is a Kconfig option that is disabled on default, just like the
crc32 init selftests in order to not unnecessary slow down boot process.
We also refactored out prandom_seed_very_weak() as it's now used in
multiple places in order to reduce redundant code.

GSL code we used for generating test cases:

  int i, j;
  srand(time(NULL));
  for (i = 0; i < 100; ++i) {
    int iteration = 500 + (rand() % 500);
    gsl_rng_default_seed = rand() + 1;
    gsl_rng *r = gsl_rng_alloc(gsl_rng_taus113);
    printf("\t{ %lu, ", gs