[RFC][Target] Add a new triple called Triple::csky

Before upstream a new target called CSKY, make a new triple of that called Triple::csky.
For now, it's a 32-bit little endian target and the detail can be referred at D86269.

This is the split part of D86269, which add a new target called CSKY.

Differential Revision: https://reviews.llvm.org/D86505