Swift: Updating Swift package for linux configuration

[Sombre-Osmoze]

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• We need to ignore Apple platforms frameworks (Accelerate and Metal) when building the project on Linux
• We need to define _GNU_SOURCE for certains function declaration (in sched.h)

[sindresorhus]

The Package.swift here closely follows https://github.com/ggerganov/llama.cpp/blob/master/Package.swift, so this should be done there first.