This introduce basic tablegen infra such as CSKY{InstrFormats,InstrInfo,RegisterInfo,}.td. For now, only add instruction definitions for basic CSKY ISA operations, and the instruction format and register info are almost complete.
Our initial target is a working MC layer rather than codegen, so appropriate SelectionDAG patterns will come later.
LLVM recently deprecated LLVMBuild, some of the functionalities were migrated to CMake. For example, LibraryGroup is replaced by add_llvm_component_group CMake function. You can take a look at how other targets did the migration