[clang] [llvm] [RISCV] Add partial support for -fzero-call-used-regs (PR #194883)

[Lucas Chollet via cfe-commits]

LucasChollet wrote:

@nickdesaulniers @bwendling Would you mind taking a look? As you were both involved in adding support for `-fzero-call-used-regs` to other architectures.

https://github.com/llvm/llvm-project/pull/194883