Add .cargo/config.toml setting CFLAGS_riscv32im_risc0_zkvm_elf to
"-march=rv32im -nostdlib". This prevents cc-rs from injecting
macOS-specific flags (-arch arm64, -mmacosx-version-min) into the
riscv32-unknown-elf-gcc cross-compiler when building C dependencies
(e.g. ring) for the guest target, which causes build failures on
macOS 26 Tahoe.
