riscv/cmake: protected mode issue

Here are issues encountered with PROTECTED mode risc-v device after commit `741de4b45055a7faec0d` applied:

```
$ rm -rf * && cmake ~/Projects/Nuttx/nuttx -DBOARD_CONFIG=canmv230/pnsh -G Ninja
-- Initializing NuttX
-- Board: canmv230
-- Config: pnsh
-- Appdir: /home/yf/Projects/Nuttx/nuttx/../apps
-- The C compiler identification is GNU 10.2.0
-- The CXX compiler identification is GNU 10.2.0
-- The ASM compiler identification is GNU
-- Found assembler: /usr/bin/riscv64-unknown-elf-gcc
riscv64-unknown-elf-gcc: error: -Wstrict-prototypes -fno-common -Wall -Wshadow -Wundef -Wno-attributes -Wno-unknown-pragmas: No such file or directory
-- Configuring done
CMake Error at cmake/nuttx_add_library.cmake:97 (add_library):
Cannot find source file:

/home/yf/Projects/Nuttx/nuttx/libs/libc/spawn/lib_task_spawn.c
Call Stack (most recent call first):
cmake/nuttx_add_library.cmake:129 (nuttx_add_system_library)
libs/libc/CMakeLists.txt:20 (nuttx_add_kernel_library)

CMake Error at CMakeLists.txt:422 (add_executable):
No SOURCES given to target: nuttx_user

CMake Error at cmake/nuttx_add_library.cmake:135 (add_library):
No SOURCES given to target: kc
Call Stack (most recent call first):
libs/libc/CMakeLists.txt:20 (nuttx_add_kernel_library)

CMake Generate step failed. Build files cannot be regenerated correctly.
```