x86: PIE executable randomization

main executable of (specially compiled/linked -pie/-fpie) ET_DYN binaries
onto a random address (in cases in which mmap() is allowed to perform a
randomization).

The code has been extraced from Ingo's exec-shield patch
http://people.redhat.com/mingo/exec-shield/

[akpm@linux-foundation.org: fix used-uninitialsied warning]
[kamezawa.hiroyu@jp.fujitsu.com: fixed ia32 ELF on x86_64 handling]

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Roland McGrath <roland@redhat.com>
Cc: Jakub Jelinek <jakub@redhat.com>
Cc: "Luck, Tony" <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>