CMake Support for The JH7110 Board

I need to add cmake support because Kconfig refuses to use LLVM/Clang for this board.

I am considering making a PR for this, however I ran into an issue with cmake as well. Removing this line https://github.com/apache/nuttx/blob/master/CMakeLists.txt#L401 let me use a custom clang executable for the toolchain. Without it, CMake tries to find riscv64-unknown-elf-gcc which I don't have. I'd rather not build gcc multiple times for different targets - that's why I'm using LLVM.

Start with CMakeLists.txt around line 401 and inspect how Kconfig selects the JH7110 toolchain. Trace the existing board toolchain configuration and determine how custom LLVM/Clang executables are expected to be selected instead of riscv64-unknown-elf-gcc. Done means the board can use CMake with the requested Clang toolchain without requiring repeated GCC builds.

Written by the indexing model from the issue text.