rand32_timer: make it more random-like for tests to pass

I get a high failure rate for tests/kernel/mem_protect/stack_random
because the default rand32_timer used with QEMU is just too mediocre.
Make it more random looking.

Reference: https://nuclear.llnl.gov/CNP/rng/rngman/node4.html



Signed-off-by: Nicolas Pitre <npitre@baylibre.com>