ggml-cpu: Conditionally enable power11 backend based on compiler support (#24687)
* ggml: Conditionally enable power11 backend based on compiler support
Guard POWER11 backend creation behind a compiler flag check for -mcpu=power11. This avoids build failures on current GCC/Clang toolchains while preserving forward compatibility once POWER11 support becomes available.
* Update CMakeLists.txt
ggml-cpu: Use -mcpu=power10 for P10 and P11