mmap randomization for executable mappings on 32-bit

mmap randomization for executable mappings on 32-bit

This code is originally from Ingo, with some later rebasing and
fixes to respect all the randomization-disabling knobs.

It provides a new address randomization algorithm that's preferable
for executable mappings in 32-bit processes.

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