build{libcompat}: Pass UNIVERSE_TOOLCHAIN_PATH to the _lc_build-tools submake

This fixes make UNIVERSE_TOOLCHAIN=yes with libcompats.

Reviewed by:	jrtc27
Differential Revision:	https://reviews.freebsd.org/D41535

Makefile.libcompat

@@ -63,6 +63,7 @@ build${libcompat}: .PHONY
 	    DESTDIR= \
 	    OBJTOP=${LIB${_LIBCOMPAT}_OBJTOP} \
 	    OBJROOT='$${OBJTOP}/' \
+	    UNIVERSE_TOOLCHAIN_PATH=${UNIVERSE_TOOLCHAIN_PATH} \
 	    MAKEOBJDIRPREFIX= \
 	    -DNO_LINT \
 	    -DNO_CPU_CFLAGS \