Add riscv build

It looks like one can cross-compile for riscv using:
```
cmake -B build \
-DCMAKE_SYSTEM_NAME=Linux \
-DCMAKE_SYSTEM_PROCESSOR=riscv64 \
-DCMAKE_C_COMPILER=riscv64-linux-gnu-gcc \
-DCMAKE_CXX_COMPILER=riscv64-linux-gnu-g++ \
-DGGML_RVV=ON
cmake --build build --config Release
```

Start by running the supplied CMake configure and build commands, then inspect the repository's existing build configuration to determine where RISC-V support belongs. Done means the project configures and builds successfully for riscv64 with GGML_RVV enabled.

Written by the indexing model from the issue text.