Add missing package libc6-riscv64-cross

The error we would get on CI is:
```
/usr/bin/riscv64-linux-gnu-gcc --sysroot=/usr/riscv64-linux-gnu    -o CMakeFiles/cmTC_eb936.dir/testCCompiler.c.o -c /pytorch/executorch/cmake-out/CMakeFiles/CMakeScratch/TryCompile-o3upsB/testCCompiler.c
Linking C executable cmTC_eb936
/opt/conda/envs/py_3.10/lib/python3.10/site-packages/cmake/data/bin/cmake -E cmake_link_script CMakeFiles/cmTC_eb936.dir/link.txt --verbose=1
/usr/lib/gcc-cross/riscv64-linux-gnu/11/../../../../riscv64-linux-gnu/bin/ld: cannot find /usr/riscv64-linux-gnu/lib/libc.so.6 inside /usr/riscv64-linux-gnu
/usr/lib/gcc-cross/riscv64-linux-gnu/11/../../../../riscv64-linux-gnu/bin/ld: cannot find /usr/riscv64-linux-gnu/lib/libc_nonshared.a inside /usr/riscv64-linux-gnu
/usr/lib/gcc-cross/riscv64-linux-gnu/11/../../../../riscv64-linux-gnu/bin/ld: cannot find /usr/riscv64-linux-gnu/lib/ld-linux-riscv64-lp64d.so.1 inside /usr/riscv64-linux-gnu
collect2: error: ld returned 1 exit status
```

Author: luhenry

examples/riscv/setup.sh

@@ -25,6 +25,8 @@ ${SUDO} apt-get install -y --no-install-recommends \
     gcc-riscv64-linux-gnu \
     g++-riscv64-linux-gnu \
     binutils-riscv64-linux-gnu \
+    libc6-riscv64-cross \
+    libc6-dev-riscv64-cross \
     qemu-user-static
 
 riscv64-linux-gnu-gcc --version | head -n1