[llvm] [RFC][RISCV] Support the large code model. (PR #70308)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Kindly ping. [spec](https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/388) seems almost being ready.

I believe the spec contains a lot of unresolved discussions and I would not call it "almost being ready".

On the GCC side, `RISC-V: Support -mcmodel=large.` landed on Dec 20.

https://github.com/llvm/llvm-project/pull/70308