Enable ASLR by default for 64-bit executables

Address Space Layout Randomization (ASLR) is an exploit mitigation
technique implemented in the majority of modern operating systems.
It involves randomly positioning the base address of an executable
and the position of libraries, heap, and stack, in a process's address
space. Although over the years ASLR proved to not guarantee full OS
security on its own, this mechanism can make exploitation more difficult.

Tests on the tier 1 64-bit architectures demonstrated that the ASLR is
stable and does not result in noticeable performance degradation,
therefore it should be safe to enable this mechanism by default.
Moreover its effectiveness is increased for PIE (Position Independent
Executable) binaries. Thanks to commit 9a227a2fd642 ("Enable PIE by
default on 64-bit architectures"), building from src is not necessary
to have PIE binaries. It is enough to control usage of ASLR in the
OS solely by setting the appropriate sysctls.

This patch toggles the kernel settings to use address map randomization
for PIE & non-PIE 64-bit binaries. It also disables SBRK, in order
to allow utilization of the bss grow region for mappings. The latter
has no effect if ASLR is disabled, so apply it to all architectures.

As for the drawbacks, a consequence of using the ASLR is more
significant VM fragmentation, hence the issues may be encountered
in the systems with a limited address space in high memory consumption
cases, such as buildworld. As a result, although the tests on 32-bit
architectures with ASLR enabled were mostly on par with what was
observed on 64-bit ones, the defaults for the former are not changed
at this time. Also, for the sake of safety keep the feature disabled
for 32-bit executables on 64-bit machines, too.

The committed change affects the overall OS operation, so the
following should be taken into consideration:

• Address space fragmentation.
• A changed ABI due to modified layout of address space.
• More complicated debugging due to:
  • Non-reproducible address space layout between runs.
  • Some debuggers automatically disable ASLR for spawned processes, making target's environment different between debug and non-debug runs.

In order to confirm/rule-out the dependency of any encountered issue
on ASLR it is strongly advised to re-run the test with the feature
disabled - it can be done by setting the following sysctls
in the /etc/sysctl.conf file:
kern.elf64.aslr.enable=0
kern.elf64.aslr.pie_enable=0

Co-developed by: Dawid Gorecki <dgr@semihalf.com>
Reviewed by: emaste, kib
Obtained from: Semihalf
Sponsored by: Stormshield
MFC after: 1 month
Differential revision: https://reviews.freebsd.org/D27666

(cherry picked from commit b014e0f15bc73d80ef49b64fd1f8c29f469467cb)