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