[RPC] Update build support for cross compiling apps/cpp_rpc with OpenCL (#6229)

[RPC] Update build support for cross compiling apps/cpp_rpc with OpenCL (#6229)

* Standardize support for building and cross compiling apps/cpp_rpc.
* Add cmake coverage for building the C++ RPC server binary
  and update documentation.
* Add support for linking against custom OpenCL SDK employing
  a custom find_opencl macro. This can be useful when cross
  compiling with a custom OpenCL device driver.

* Update OpenCL related documentation.

* Add embedded linux build instructions to apps/cpp_rpc/README.md and
ensure pthread is linked against when OS=Linux is defined. Remove
outdated apps/cpp_rpc/Makefile.