Add cmake/riscv-none-elf-rv32imc.cmake targeting the xPack
riscv-none-elf-gcc toolchain with rv32imc_zicsr/ilp32 ABI for
bare-metal CORE-V MCU builds.
Update cmake/riscv32-unknown-elf-rv32imc.cmake to correctly target
rv32gc/ilp32d (matching riscv-collab riscv32-elf ABI) for QEMU
regression tests.
Update cmake/riscv64-gcc-rv32imc.cmake compat alias to include
the new riscv-none-elf-rv32imc.cmake.
Update CORE-V MCU example_build: build.sh exports xPack PATH,
install_deps.sh downloads xPack 15.2.0-1, README.md reflects new
toolchain name/path.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
