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.
$$(TLIB$(1)_T_$(2)_H_$(3))/stamp.$(4): \
$$(CRATEFILE_$(4)) \
$$(CRATE_FULLDEPS_$(1)_T_$(2)_H_$(3)_$(4)) \
+ $$(LLVM_CONFIG_$(2)) \
$$(TSREQ$(1)_T_$(2)_H_$(3)) \
| $$(TLIB$(1)_T_$(2)_H_$(3))/
@$$(call E, rustc: $$(@D)/lib$(4))