To address the "offset2lib" ASLR weakness[1], this separates ET_DYN
ASLR from mmap ASLR, as already done on s390. The architectures
that are already randomizing mmap (arm, arm64, mips, powerpc, s390,
and x86), have their various forms of arch_mmap_rnd() made available
via the new CONFIG_ARCH_HAS_ELF_RANDOMIZE. For these architectures,
arch_randomize_brk() is collapsed as well.

This is an alternative to the solutions in:
https://lkml.org/lkml/2015/2/23/442

I've been able to test x86 and arm, and the buildbot (so far) seems
happy with building the rest.

Thanks!

-Kees

[1] http://cybersecurity.upv.es/attacks/offset2lib/offset2lib.html

v3:
- split change on a per-arch basis for easier review
- moved PF_RANDOMIZE check out of per-arch code (ingo)
v2:
- verbosified the commit logs, especially 4/5 (akpm)