exec-randomization: brk away from exec rand area

exec-randomization: brk away from exec rand area

This is a fix for the NX emulation patch to force the brk area well
outside of the exec randomization area to avoid future allocation or brk
growth collisions. Normally this isn't a problem, except when the text
region has been loaded from a PIE binary and the CS limit can't be put
just above bss.

A test-case that will show failures without this patch can be found here:
http://bazaar.launchpad.net/~ubuntu-bugcontrol/qa-regression-testing/master/annotate/head%3A/scripts/kernel-aslr-collisions/explode-brk.c

Signed-off-by: Kees Cook <kees.cook@canonical.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>