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
libraries libXXX_pie.a.

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
Makefile rules.

Currently only dynamically linked binaries will be built as PIE.

Discussed with:	dim
Reviewed by:	kib
MFC after:	1 month
Relnotes:	Yes
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D18423

kerberos5/tools/asn1_compile/Makefile

@@ -6,6 +6,7 @@ LIBROKEN_A=	${.OBJDIR:H:H}/lib/libroken/libroken.a
 LIBADD=	vers
 LDADD=	${LIBROKEN_A}
 DPADD=	${LIBROKEN_A}
+MK_PIE:=	no
 
 SRCS=	\
 	asn1parse.y \

kerberos5/tools/slc/Makefile

@@ -6,6 +6,7 @@ LIBADD=	vers
 LDADD=  ${LIBROKEN_A}
 DPADD=  ${LIBROKEN_A}
 MAN=
+MK_PIE:=	no
 
 SRCS=	roken.h \
 	slc-gram.y \