[Feature Request] Support configurable RISC-V chip extension

[JoeBenczarski]

The problem is that not all RISC-V chip extensions are supported for all compilers.

The solution would be to add a FREERTOS_RISC_EXTENSION option to the CMake project similar to how FREERTOS_PORT is used. The CMake project would issue an error and provide valid options if the user chooses a RISC_V port but does not specify a chip extension folder name.

[chinglee-iot]

@JoeBenczarski
The PR is merged. Thank you for your contribution.