Enable PIE by default on 64-bit architectures

This patch adds Position Independent Executables (PIE)
flags for building OS. It allows to enable the ASLR
feature based only on the sysctl knobs, without
need to rebuild the image. Tests showed that
no problems with stability / performance degradation
were seen when using PIEs with ASLR disabled.

The change is limited only for 64-bit architectures.

Use bsd.opts.mk instead of the src.opts.mk in order
to satisfy all build dependencies related to MK_PIE.

Reviewed by: emaste, imp
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D28328