[clang] [RISCV] Allow -mcmodel= to accept large for RV64 (PR #107817)

[Jim Lin via cfe-commits]

tclin914 wrote:

> What I am missing is:
> 
> * adjustments in `clang/lib/Basic/Targets/RISCV.cpp` to emit the macro `__riscv_cmodel_large`
> * new tests in `clang/test/Preprocessor/riscv-cmodel.c`
> 
> Related PRs:
> 
> * [Add __riscv_cmodel_large define for large code model riscv-non-isa/riscv-c-api-doc#86](https://github.com/riscv-non-isa/riscv-c-api-doc/pull/86)
> * [Add large code model riscv-non-isa/riscv-toolchain-conventions#59](https://github.com/riscv-non-isa/riscv-toolchain-conventions/pull/59)

A follow-up PR #108131 for `__riscv_cmodel_large`

https://github.com/llvm/llvm-project/pull/107817