[LoongArch 4/6] Add basic tablegen infra for LoongArch

This patch introduces basic tablegen infra such as
LoongArch{InstrFormats,InstrInfo,RegisterInfo,CallingConv,}.td.

For now, only add instruction definitions for LoongArch basic integer
operations.
Our initial target is a working MC layer rather than codegen,
so appropriate SelectionDAG patterns will come later.

Differential revision: https://reviews.llvm.org/D115861