Fallback to Vulkan for whisper.cpp in case of hipblas builds.

[TimoStolz]

Is your feature request related to a problem? Please describe.
When using a hipblas build, I'm always frustrated that whisper.cpp is using my CPU instead of my GPU. Unfortunately, whisper.cpp does not support ROCm or hipblas.

Describe the solution you'd like
However, my AMD GPU does not only support ROCm/hipblas, but also Vulkan. Just like whisper.cpp.

Describe alternatives you've considered
At the moment, I'm using a full Vulkan build to be compatible with whisper.cpp. However, I'd prefer to use ROCm when available, else fallback to Vulkan.