Use a XOR cipher instead of byte shuffling to protect against bad seeds.

libgfortran:
2016-08-19  Janne Blomqvist  <jb@gcc.gnu.org>

        * intrinsics/random.c (xor_keys): New array with "secret" keys.
        (scramble_seed): XOR given seed with xor_keys array rather than
        shuffling bytes.
        (unscramble_seed): Remove function.
        (random_seed_i4): Use new scramble_seed.
        (random_seed_i8): Likewise.

frontend:
2016-08-19  Janne Blomqvist  <jb@gcc.gnu.org>

        * intrinsics.texi (RANDOM_NUMBER): Remove reference to
        init_random_seed in example.
        (RANDOM_SEED): Remove warning to not set all seed values to 0.

From-SVN: r239613