Make PKGBUILD architecture-aware

Christer Solskogenrequested to merge
solskogen/rust:aarch64 into main
  • Move x86_64-specific packages to pkgname_x86_64 array
  • Add conditional logic for architecture-specific build configurations
  • Extract license installation to _install_licenses() helper function
  • Support building on non-x86_64 architectures

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