readme : partial OpenCL GPU support via CLBlast (#863)

readme : partial OpenCL GPU support via CLBlast (#863)

* ggml : CLBlast support as in llama.cpp

Building with CLBlast speeds up whisper.cpp ~2x on low end / older AMD APUs (CPU with integrated GPU) such as the A9.

Usage:
WHISPER_CLBLAST=1 make

* CMake/Makefile : CLBlast support as in llama.cpp

Building with CLBlast speeds up whisper.cpp ~2x on low end / older AMD APUs (CPU with integrated GPU) such as the A9.

Usage:
```
Makefile:
cd whisper.cpp
WHISPER_CLBLAST=1 make

CMake:
cd whisper.cpp ; mkdir build ; cd build
cmake -DWHISPER_CLBLAST=ON  ..
make
```

* Update README.md

Added OpenCL Build Instructions

* Instruction: Partial OpenCL GPU support via CLBlast

Added build instructions and examples for Make and CMake to support OpenCL enabled GPUs.