cmake : use ggml-metal.metal from source dir to build default.metallib

[cebtenzzre]

I noticed an edge case while building the ggml-metal target with Ninja more than once. The specific steps to reproduce are:

$ cmake -B build -G Ninja -DGGML_METAL_EMBED_LIBRARY=OFF  # copies ggml-metal.metal to build/bin/ 
$ ninja -C build ggml-metal  # succeeds, removes build/bin/ggml-metal.metal
$ ninja -C build clean  # removes generated default.metallib, but ggml-metal.metal is still gone
$ ninja -C build ggml-metal  # fails to find build/bin/ggml-metal.metal
ninja: Entering directory `build'
[1/1] Compiling Metal kernels
FAILED: bin/default.metallib /Users/jared/src/forks/llama.cpp/build/bin/default.metallib
cd /Users/jared/src/forks/llama.cpp/build/ggml/src && xcrun -sdk macosx metal -O3 -c /Users/jared/src/forks/llama.cpp/build/bin/ggml-metal.metal -o /Users/jared/src/forks/llama.cpp/build/bin/ggml-metal.air && xcrun -sdk macosx metallib /Users/jared/src/forks/llama.cpp/build/bin/ggml-metal.air -o /Users/jared/src/forks/llama.cpp/build/bin/default.metallib && rm -f /Users/jared/src/forks/llama.cpp/build/bin/ggml-metal.air && rm -f /Users/jared/src/forks/llama.cpp/build/bin/ggml-common.h && rm -f /Users/jared/src/forks/llama.cpp/build/bin/ggml-metal.metal
metal: error: no such file or directory: '/Users/jared/src/forks/llama.cpp/build/bin/ggml-metal.metal'
metal: error: no input files
ninja: build stopped: subcommand failed.

This does not happen with make, since it notices the missing files and runs cmake again to regenerate the byproducts (which are created at configure time via configure_file yet deleted at build time). This still happens after this PR, which is not ideal, but I don't fully understand why we are deleting build artifacts based on the target you select. @ggerganov Are the rm commands important here?

This also does not happen with the default of GGML_METAL_EMBED_LIBRARY=ON, since it does not rm the input files afterwards.

The simple fix is to use ggml-metal.metal from the source dir to build default.metallib, which exists regardless of whether you have previously run the target.

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[ggerganov]

@ggerganov Are the rm commands important here?

I am not really sure what was the reason to delete the files. Maybe we should remove the rm statements and see if it works as expected.