[clang] [flang] [flang][RISCV][WIP] Add target-abi ModuleFlag. (PR #126188)

[via cfe-commits]

https://github.com/jeanPerier approved this pull request.

I do not know much the target-abi LLVM module flag, but this looks reasonable.

https://github.com/llvm/llvm-project/pull/126188