Prefer MK_SSP=no to SSP_CFLAGS=

It is more idiomatic. CFLAGS is only augmented with $SSP_CFLAGS when
$MK_SSP != "no".

Reviewed by:	imp
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D31401

Makefile.libcompat

@@ -99,7 +99,7 @@ build${libcompat}: .PHONY
 	${_+_}cd ${.CURDIR}/${_dir}; \
 	    WORLDTMP=${WORLDTMP} \
 	    MAKEFLAGS="-m ${.CURDIR}/tools/build/mk ${.MAKEFLAGS}" \
-	    ${MAKE} SSP_CFLAGS= DESTDIR= \
+	    ${MAKE} DESTDIR= \
 	    OBJTOP=${LIBCOMPAT_OBJTOP} \
 	    OBJROOT='$${OBJTOP}/' \
 	    MAKEOBJDIRPREFIX= \
@@ -109,6 +109,7 @@ build${libcompat}: .PHONY
 	    MK_ASAN=no \
 	    MK_CTF=no \
 	    MK_RETPOLINE=no \
+	    MK_SSP=no \
 	    MK_UBSAN=no \
 	    MK_WERROR=no \
 	    ${_t}