Add LLVM 22 and Halide 21 prebuilts

[awakecoding]

Summary

• drop LLVM 18 and add LLVM 22.1.4 prebuilts
• drop Halide 18 and add Halide 21.0.0 prebuilts
• refresh LLVM/Halide patch sets and version conditionals
• fix Linux aarch64 dependency setup and Windows static zlib/libxml2 discovery for LLVM and Halide workflows

Validation

• LLVM prebuilt workflow: https://github.com/awakecoding/llvm-prebuilt/actions/runs/25380393778
• Halide prebuilt workflow: https://github.com/awakecoding/llvm-prebuilt/actions/runs/25393560823