Avoid passing -L "" during cross-compilation.

LLVM_LIBDIR_<triple> is only defined for host triples, not target triples.

FWIW, the same is true for LLVM_STDCPP_RUSTFLAGS_<triple>, where we
explicitly define it as empty when --enable-llvm-static-stdcpp is not
specified, but it's still undefined for cross-compiled triples.

mk/main.mk

@@ -290,6 +290,7 @@ LLVM_VERSION_$(1)=$$(shell "$$(LLVM_CONFIG_$(1))" --version)
 LLVM_BINDIR_$(1)=$$(shell "$$(LLVM_CONFIG_$(1))" --bindir)
 LLVM_INCDIR_$(1)=$$(shell "$$(LLVM_CONFIG_$(1))" --includedir)
 LLVM_LIBDIR_$(1)=$$(shell "$$(LLVM_CONFIG_$(1))" --libdir)
+LLVM_LIBDIR_RUSTFLAGS_$(1)=-L "$$(LLVM_LIBDIR_$(1))"
 LLVM_LIBS_$(1)=$$(shell "$$(LLVM_CONFIG_$(1))" --libs $$(LLVM_COMPONENTS))
 LLVM_LDFLAGS_$(1)=$$(shell "$$(LLVM_CONFIG_$(1))" --ldflags)
 # On FreeBSD, it may search wrong headers (that are for pre-installed LLVM),

mk/target.mk

@@ -84,7 +84,7 @@ $$(TLIB$(1)_T_$(2)_H_$(3))/stamp.$(4): \
 	    $$(subst @,,$$(STAGE$(1)_T_$(2)_H_$(3))) \
 		$$(RUST_LIB_FLAGS_ST$(1)) \
 		-L "$$(RT_OUTPUT_DIR_$(2))" \
-		-L "$$(LLVM_LIBDIR_$(2))" \
+		$$(LLVM_LIBDIR_RUSTFLAGS_$(2)) \
 		$$(LLVM_STDCPP_RUSTFLAGS_$(2)) \
 		$$(RUSTFLAGS_$(4)) \
 		--out-dir $$(@D) \

mk/tests.mk

@@ -372,7 +372,7 @@ $(3)/stage$(1)/test/$(4)test-$(2)$$(X_$(2)): \
 	$(Q)CFG_LLVM_LINKAGE_FILE=$$(LLVM_LINKAGE_PATH_$(3)) \
 	    $$(subst @,,$$(STAGE$(1)_T_$(2)_H_$(3))) -o $$@ $$< --test \
 		-L "$$(RT_OUTPUT_DIR_$(2))" \
-		-L "$$(LLVM_LIBDIR_$(2))" \
+		$$(LLVM_LIBDIR_RUSTFLAGS_$(2)) \
 		$$(RUSTFLAGS_$(4))
 
 endef