MIPS: Enable heap randomization.

Based somewhat on the PPC implementation.

32-bit processes have the heap randomized in an 8MB space, 256MB for
64-bit processes.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1479/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>