HBSD: Disable BIND_NOW for libc and the RTLD

ifuncs can be incompatible with BIND_NOW in certain cases[1]. This would
be the case for libc and the RTLD. As such, we should disable BIND_NOW
for both libc and the RTLD.

Unfortunately, using ifuncs in libc opens up a potential attack vector
due to libc's PLT/GOT not using full RELRO (RELRO + BIND_NOW).

From the ifunc documentation[1]:

"When LD_BIND_NOW=1 or -Wl,z,now is in effect symbols must be
immediately resolved at startup. In cases where an external function
call depends needs to be made that may fail if such a call has not been
initialized yet (PLT-based relocation which is processed later). For
example calling strlen in an IFUNC resolver built with -Wl,z,now may
lead to a segfault because the PLT is not yet resolved."

[1]: https://sourceware.org/glibc/wiki/GNU_IFUNC

Signed-off-by:	Shawn Webb <shawn.webb@hardenedbsd.org>
Sponsored-by:	SoldierX
github-issue:	#357
MFC-to:		12-STABLE

lib/libc/amd64/Makefile.inc

@@ -7,3 +7,5 @@
 GDTOASRCS+=strtorx.c
 SRCS+=machdep_ldisx.c
 SYM_MAPS+=${LIBC_SRCTOP}/amd64/Symbol.map
+
+MK_BIND_NOW=	no

libexec/rtld-elf/amd64/Makefile.inc

@@ -1,5 +1,7 @@
 # $FreeBSD$
 
+MK_BIND_NOW=	no
+
 CFLAGS+=	${CFLAGS_NO_SIMD} -msoft-float
 # Uncomment this to build the dynamic linker as an executable instead
 # of a shared library: