Subzero: Minor Makefile fix.

The problem showed up after the link step failed, in which case $(OBJDIR)/llvm2ice was deleted but the ./llvm2ice symlink still existed.  A subsequent "make check-lit" or "make check" would fail, so the basic "make" would have to be done first.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/887873002
diff --git a/Makefile.standalone b/Makefile.standalone
index e906d56..e246a60 100644
--- a/Makefile.standalone
+++ b/Makefile.standalone
@@ -182,7 +182,7 @@
 $(OBJDIR)/unittest: $(OBJDIR)
 	@mkdir -p $@
 
-check-lit: llvm2ice make_symlink
+check-lit: $(OBJDIR)/llvm2ice make_symlink
 	LLVM_BIN_PATH=$(LLVM_BIN_PATH) \
 	$(LLVM_SRC_PATH)/utils/lit/lit.py -sv tests_lit