Fix lcg_rand_u32() to return 32 random bits.

Fix lcg_rand_u32() to return 32 random bits.

The lcg_rand() function only returns 15 random bits, so lcg_rand_u32()
would always have 0 in bit 31 and bit 15. Fix that by calling
lcg_rand() three times, to generate 15, 15, and 2 random bits
respectively.

V2: Use the 10/11 most significant bits from the 3 lcg results and mix
them with the low ones from the adjacent one, as suggested by Andrea
Canciani.