The RUST_TARGET_STAGE_N rule uses LLVM_LIBDIR_RUSTFLAGS_<target-triple>,
which expands to -L "$(llvm-config --libdir)" when the target-triple is
also a host-triple. Rather than expand to -L "" if llvm-config has not yet
been built, add a dependency on the target llvm-config.
When the target-triple is not a host-triple, the new LLVM_CONFIG_$(2)
dependency should expand to nothing.
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