Always bring tblgen target up to date.

The copy-only-diff trick violates the makefile assumption:
Rules should bring the target up to date if any source has newer time
stamp.

Change-Id: I9301d00461f1480ffd9293b81294985e58d4cdef
diff --git a/llvm-device-build.mk b/llvm-device-build.mk
index 00f089d..83fc5ef 100644
--- a/llvm-device-build.mk
+++ b/llvm-device-build.mk
@@ -73,7 +73,5 @@
 	-I $(LLVM_ROOT_PATH)/device/include	\
 	-I $(LLVM_ROOT_PATH)/lib/Target	\
     -gen-$(strip $(1)) \
-    -o $@.tmp $<
-$(hide) $(LLVM_ROOT_PATH)/copy-diff.py $@.tmp $@
-$(hide) $(RM) $@.tmp
+    -o $@ $<
 endef