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

[Christoph Müllner via cfe-commits]

https://github.com/cmuellner commented:

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:
* https://github.com/riscv-non-isa/riscv-c-api-doc/pull/86
* https://github.com/riscv-non-isa/riscv-toolchain-conventions/pull/59


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