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
Select the arch_variant_cflags for the 2nd arch.

Change-Id: Id2f9d7073a4aae3ba0fe5e5464045761f4d42b4e