We can't build those three with MKSANITIZER, they are used too early.

lib/libc/Makefile.inc

@@ -1,4 +1,4 @@
-#	$NetBSD: Makefile.inc,v 1.18 2015/08/28 11:45:02 joerg Exp $
+#	$NetBSD: Makefile.inc,v 1.19 2018/06/09 22:41:55 christos Exp $
 #	@(#)Makefile	8.2 (Berkeley) 2/3/94
 #
 # All library objects contain sccsid strings by default; they may be
@@ -17,6 +17,7 @@
 # The Hesiod functions are always in libc. To choose that getpwent() and friends
 # actually call the Hesiod functions, put -DHESIOD on the CPPFLAGS line below.
 
+NOSANITIZER=	# defined
 USE_FORT?=	yes
 
 USE_SHLIBDIR=	yes

lib/libpthread/Makefile

@@ -1,6 +1,7 @@
-#	$NetBSD: Makefile,v 1.88 2018/02/25 18:51:18 chs Exp $
+#	$NetBSD: Makefile,v 1.89 2018/06/09 22:41:55 christos Exp $
 #
 
+#NOSANITIZER=	# defined
 WARNS?=	5
 LIB=	pthread
 

libexec/ld.elf_so/Makefile

@@ -1,8 +1,8 @@
-#	$NetBSD: Makefile,v 1.137 2017/06/19 11:57:01 joerg Exp $
+#	$NetBSD: Makefile,v 1.138 2018/06/09 22:41:55 christos Exp $
 #
 # NOTE: when changing ld.so, ensure that ldd still compiles.
 #
-
+NOSANITIZER=	# defined
 WARNS?=4
 
 LDELFSO_MACHINE_ARCH?= ${MACHINE_ARCH}