[CSKY] Add CSKY 16-bit instruction format and encoding

CSKY is a ARCH which supports mixture of 16-bit and 32-bit instructions natively,
and there is not an indivual predictor or feature to enable/disable 16-bit instruction.
So I think it's better to add 16-bit instruction early, and naturally to use 16-bit and 32-bit instructions.

Differential Revision: https://reviews.llvm.org/D112919