cmake: rename rv32 toolchain file to riscv32-unknown-elf-rv32imc.cmake (#537)

Renamed cmake/riscv64-gcc-rv32imc.cmake to the accurate name
cmake/riscv32-unknown-elf-rv32imc.cmake and switch the compiler
from riscv64-unknown-elf-gcc to riscv32-unknown-elf-gcc.

The riscv-collab riscv64-elf toolchain has no rv32 multilib and
will fail to link soft-float and integer helpers (__clzsi2, __muldf3,
etc.) when building for -march=rv32imc_zicsr -mabi=ilp32.  The
dedicated riscv32-unknown-elf-gcc (riscv-collab riscv32-elf release,
installed to /opt/riscv by scripts/install_riscv.sh) ships the correct
native rv32/ilp32 libgcc — analogous to arm-none-eabi-gcc for Cortex-M.

The old filename is kept as a two-line compatibility alias that includes
the new file, so any out-of-tree users who hardcode the old path still work.

Also update:
- core_v_mcu/build.sh: reference new cmake filename
- core_v_mcu/README.md: update prerequisites table and toolchain docs

Co-authored-by: Copilot <[email protected]>

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