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

[Alex Bradbury via llvm-commits]

asb wrote:

It seems to me that we're some way from agreement and consensus on the [spec](https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/388), so I understand this patch is here for a proof of concept to accompany that discussion and to elicit early feedback/review on the implementation approach (which is a great contribution - thanks!). But I wanted to explicitly note that I think we shouldn't look to review and merge this until there's clear agreement on the spec.

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