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.

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