Refactor llvm_config.mk and support the 2nd arch

1. Following the setup of gcc in build/core/combo/,
we added the [HOST|TARGET]_<arch>.mk clang config files,
and load only the configs needed by the current product.
2. Added support for the 2nd arch.

Change-Id: I2a383418a9688a050b39492f8e489d40eeeb5f2d
diff --git a/core/config.mk b/core/config.mk
index aa64c60..a1bae47 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -455,8 +455,8 @@
   TARGET_DEFAULT_JAVA_LIBRARIES := core core-junit ext framework framework2
 endif
 
-# define llvm tools and global flags
-include $(BUILD_SYSTEM)/llvm_config.mk
+# define clang/llvm tools and global flags
+include $(BUILD_SYSTEM)/clang/config.mk
 
 # ###############################################################
 # Collect a list of the SDK versions that we could compile against