cmake/rv-virt: lacking support for `NSH_SYMTAB` and `ALLSYMS`

For the `rv-virt:nsh` target, make approach works well, but CMake build leads to undefined symbols for `g_symtab` etc. It seems that `defconfig` has `CONFIG_ALLSYMS` off and `CONFIG_NSH_SYMTAB` on.

Further tests show that:

- When `CONFIG_NSH_SYMTAB` is on and `CONFIG_ALLSYMS` is off, Make build works but CMake fails.
- When both `CONFIG_NSH_SYMTAB` and `CONFIG_ALLSYMS` are off, CMake build works.
- When both `CONFIG_NSH_SYMTAB` and `CONFIG_ALLSYMS` are on, Makefile build works out a larger binary but CMake build leads to undefined `g_symtab` etc.

Is this issue in current CMake system or something else?