metal : try cwd for ggml-metal.metal if bundle lookup fails (#3793)

* Try cwd for ggml-metal if bundle lookup fails

When building with `-DBUILD_SHARED_LIBS=ON -DLLAMA_METAL=ON -DLLAMA_BUILD_SERVER=ON`,
`server` would fail to load `ggml-metal.metal` because `[bundle pathForResource:...]`
returns `nil`.  In that case, fall back to `ggml-metal.metal` in the cwd instead of
passing `null` as a path.

Follows up on #1782

* Update ggml-metal.m

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>