Another try at this.

For 32bit follow the 32bit implementation from Ingo -
mappings are growing down from the end of stack now
and vary randomly by 1GB.

Randomized mappings for 64bit just vary the normal mmap break
by 1TB. I didn't bother implementing full flex mmap for 64bit
because it shouldn't be needed there.

Cc: mingo@elte.hu
Signed-off-by: NAndi Kleen <ak@suse.de>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>