Use universe-toolchain config(8)

This is a temporary hack to aid with config(8) changing in r360443.
It will not work for all cases.

env PATH is used because universe-toolchain is only built when worlds
are built, and then only if clang is needed, so it may not exist.

universe-toolchain needs to be expanded to always be built, inspected to
remove non-cross-build-safe tools, used for buildworld/buildkernel,
and potentially incremental build support.

Sponsored by:	Dell EMC

Makefile

@@ -711,6 +711,7 @@ KERNCONFS!=	cd ${KERNSRCDIR}/${TARGET}/conf && \
 universe_kernconfs: universe_kernels_prologue .PHONY
 .for kernel in ${KERNCONFS}
 TARGET_ARCH_${kernel}!=	cd ${KERNSRCDIR}/${TARGET}/conf && \
+	env PATH=${HOST_OBJTOP}/tmp/legacy/bin:${PATH:Q} \
 	config -m ${KERNSRCDIR}/${TARGET}/conf/${kernel} 2> /dev/null | \
 	grep -v WARNING: | cut -f 2
 .if empty(TARGET_ARCH_${kernel})