Resolve #1935, enable specifying out-of-source platform folder with `cmake .. -DWAMR_BUILD_PLATFORM=new-os -DSHARED_PLATFORM_CONFIG=/path/to/new-os/shared_platform.cmake`
Resolve #1935, enable specifying out-of-source platform folder with `cmake .. -DWAMR_BUILD_PLATFORM=new-os -DSHARED_PLATFORM_CONFIG=/path/to/new-os/shared_platform.cmake`