metal : build metallib + fix embed path

[ggerganov]

fix #5977

This should improve the build process following the changes in #5940:

• CMake now always builds default.metallib
• Fix LLAMA_METAL_EMBED_LIBRARY (Makefile + CMake) to work with the new ggml-common.h

[giladgd]

This PR seems to have solved the issue on my end.
I suggest deleting ggml-metal.air, ggml-common.h and ggml-metal.metal from the output directory after building default.metallib since it seems that those files aren't necessary at runtime now.

[giladgd]

Compiling with LLAMA_METAL_EMBED_LIBRARY now relies on default.metallib to exist at runtime

[ggerganov]

I believe I fixed it - would appreciate another test if you have the opportunity

[giladgd]

I've tested your recent commit and now compiling with LLAMA_METAL_EMBED_LIBRARY fails at runtime with the original error:

ggml_metal_init: error: Error Domain=MTLLibraryErrorDomain Code=3 "program_source:3:10: fatal error: 'ggml-common.h' file not found
#include "ggml-common.h"
         ^~~~~~~~~~~~~~~
" UserInfo={NSLocalizedDescription=program_source:3:10: fatal error: 'ggml-common.h' file not found
#include "ggml-common.h"
         ^~~~~~~~~~~~~~~
}
llama_new_context_with_model: failed to initialize Metal backend