Add a test for mainless emit targets

Author: nagisa

src/test/run-make/empty-file/Makefile

@@ -0,0 +1,5 @@
+-include ../tools.mk
+
+all:
+	$(RUSTC) --emit=asm,llvm-bc,llvm-ir,obj,dep-info empty.rs
+	$(RUSTC) --emit=link --crate-type=rlib,dylib,staticlib empty.rs