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

[Alex Bradbury via cfe-commits]

asb wrote:

I think we want to set a define for this as well, for consistency with medany and medlow (as tested in test/Preprocessor/riscv-cmodel.c). I submitted a trivial PR to riscv-c-api-doc to add this https://github.com/riscv-non-isa/riscv-c-api-doc/pull/86 which I'd hope lands quickly.

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