libthr: disable thread-safety warnings

These warnings don't make sense for code that implements
the locking primitives.

Reviewed by:	kib
MFC after:	3 days
Sponsored by:	Dell EMC
Differential Revision:	https://reviews.freebsd.org/D10832

lib/libthr/Makefile

@@ -17,6 +17,7 @@ MK_SSP=	no
 LIB=thr
 SHLIB_MAJOR= 3
 WARNS?=	3
+NO_WTHREAD_SAFETY=1
 CFLAGS+=-DPTHREAD_KERNEL
 CFLAGS+=-I${SRCTOP}/lib/libc/include -I${.CURDIR}/thread \
 	-I${SRCTOP}/include