META_MODE: Fix clang-tblgen flip-flop building after r335707.

The build of this would have MK_LLVM_TARGET_ALL=yes for bootstrap-tools
but MK_LLVM_TARGET_ALL=no for cross-tools and thus would rebuild between
the two.

MFC after:	2 weeks
X-MFC-with:	r335707
Sponsored by:	Dell EMC

Makefile.inc1

@@ -666,6 +666,9 @@ BSARGS= 	DESTDIR= \
 BMAKE=		\
 		${BMAKEENV} ${MAKE} ${WORLD_FLAGS} -f Makefile.inc1 \
 		${BSARGS}
+.if empty(.MAKEOVERRIDES:MMK_LLVM_TARGET_ALL)
+BMAKE+=		MK_LLVM_TARGET_ALL=no
+.endif
 
 # build-tools stage
 TMAKE=		\
@@ -685,9 +688,6 @@ TMAKE=		\
 XMAKE=		${BMAKE} \
 		TARGET=${TARGET} TARGET_ARCH=${TARGET_ARCH} \
 		MK_GDB=no MK_TESTS=no
-.if empty(.MAKEOVERRIDES:MMK_LLVM_TARGET_ALL)
-XMAKE+=		MK_LLVM_TARGET_ALL=no
-.endif
 
 # kernel-tools stage
 KTMAKEENV=	INSTALL="sh ${.CURDIR}/tools/install.sh" \