Tracking issue for the LLVM 21 upgrade

[alexrp]

• Consider switching *-uefi-* targets to uefi instead of windows for LLVM 21+ #21630
• [NVPTX] Add tcgen05 alloc/dealloc intrinsics llvm/llvm-project#124961 (data layout change)
• [NVPTX] Add support for Shared Cluster Memory address space [1/2] llvm/llvm-project#135444 (data layout and address space changes)
• f2f36c4
• [SystemZ] Add support for half (fp16) llvm/llvm-project#109164
• [NVPTX] Basic support for fp128 as a storage type llvm/llvm-project#136006
• CPU features are not passed to clang when assembling #10411

• zig/test/llvm_targets.zig

  Line 242 in bb79c85

  // .{ .cpu_arch = .s390x, .os_tag = .zos, .abi = .none },

• zig/test/llvm_targets.zig

  Lines 355 to 356 in bb79c85

  	// .{ .cpu_arch = .xtensa, .os_tag = .freestanding, .abi = .none },
  	// .{ .cpu_arch = .xtensa, .os_tag = .linux, .abi = .none },

• On wasm fmt print of u64 packed struct triggers WebAssembly translation error  #20966
• [AMDGPU] Set AS8 address width to 48 bits llvm/llvm-project#139419 (data layout change)

Previous upgrade: #22014