Add WITH_PIE knob to build Position Independent Executables
Building binaries as PIE allows the executable itself to be loaded at a
random address when ASLR is enabled (not just its shared libraries).
With this change PIE objects have a .pieo extension and INTERNALLIB
MK_PIE is disabled for some kerberos5 tools, Clang, and Subversion, as
they explicitly reference .a libraries in their Makefiles. These can
be addressed on an individual basis later. MK_PIE is also disabled for
rtld-elf because it is already position-independent using bespoke
Currently only dynamically linked binaries will be built as PIE.
Discussed with: dim
Reviewed by: kib
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18423