[REFACTOR][TARGET] Cleanup backend target registration

[tqchen]

The backend reorg leaves target-kind and op registration behind generic register.cc shims, which makes the new layout harder to navigate and keeps a few obsolete target paths alive. This change keeps backend registration beside the backend definitions while preserving the existing static initialization behavior.

Backend target-kind registration units now use semantic target_kind.cc names, target builtin registration is colocated with the CUDA, Metal, and Trainium builtin definitions, the NVPTX intrinsic rules live with CUDA LLVM codegen, and the unused target/opt placeholder is removed.

[gemini-code-assist]

Code Review

This pull request refactors backend static registration across multiple targets (CUDA, Hexagon, Metal, OpenCL, ROCm, Trainium, Vulkan, and WebGPU) by renaming registration files to target_kind.cc and moving target builtin registration blocks directly into their respective target_builtin.cc files. Feedback points out a critical build issue where the relocated intrin_rule_nvptx.cc file is excluded from compilation due to non-recursive globbing in CMakeLists.txt, and suggests adding the new path to the source list.

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[gemini-code-assist]

The file src/target/llvm/intrin_rule_nvptx.cc has been renamed to src/backend/cuda/codegen/llvm/intrin_rule_nvptx.cc. However, because CMakeLists.txt only globs src/backend/cuda/codegen/*.cc (non-recursively), and cmake/modules/LLVM.cmake likely only globs src/target/llvm/*.cc, this file is now excluded from the build. To ensure the NVPTX intrinsic rules are compiled, we should add src/backend/cuda/codegen/llvm/*.cc to the CODEGEN_SRCS list.

  src/backend/cuda/codegen/llvm/*.cc
  src/backend/vulkan/codegen/target_kind.cc