feat(rocm): Add Windows support to ROCm backend

[antmikinka]

Summary

This PR updates the rocm-support branch with full Windows compatibility for the ROCm/HIP backend. The original ROCm backend was developed for Linux; this PR adapts it to work with ROCm 7.1 on Windows 11.

Changes Made

CMake Build System

• Root CMakeLists.txt: Added Windows ROCm installation paths (C:/Program Files/AMD/ROCm/7.1/bin) and environment variable hints (ROCM_PATH, ROCM_HOME) for HIP compiler discovery
• mlx/backend/rocm/CMakeLists.txt:
  • Guarded GCC/Linux-specific include path discovery with if(NOT WIN32)
  • Added Windows ROCm include directory fallback paths
  • Platform-specific object file extensions (.obj on Windows, .o on Linux)
  • Removed -fPIC on Windows (not needed for DLLs)
  • Guarded -parallel-jobs clang-specific flag on Windows
  • Platform-specific static library naming (.lib on Windows, .a on Linux)
  • Platform-specific archiver command (/OUT: for MSVC, rcs for Unix)
  • Unified ROCm library search paths with Windows defaults

Source Code

• allocator.cpp:
  • Made #include <unistd.h> conditional on #ifndef _WIN32
  • Implemented cross_platform_clzll() using _BitScanReverse64 on MSVC and __builtin_clzll elsewhere
• jit_module.cpp:
  • Replaced POSIX stderr suppressor with Windows equivalent (_dup/_dup2/_open("nul"))
  • Added Windows ROCm path detection via ProgramFiles env var and multiple version fallbacks

Test Infrastructure

• rocm_skip.py: Added skip entries for Metal-specific kernel tests (test_export_import_custom_kernel, test_custom_kernel_caching)
• Merged latest main (31 commits) including critical Windows JIT compiler support (5d1c0e4c)

Commits

1. 16c411f8 — Merge origin/main into rocm-support
2. 36f37be3 — feat(rocm): Add Windows support to CMake build files
3. 3da1e3f2 — feat(rocm): Add Windows support to ROCm backend source files
4. 3239866e — fix(rocm): Address quality review findings for Windows build
5. 2adb9d93 — fix(rocm): Add ROCm skip entries for Metal-specific kernel tests

Environment

• ROCm 7.1 (HIP 7.1.51803) on Windows 11 Pro
• Installed at C:\Program Files\AMD\ROCm\7.1\

Notes

• This PR targets the rocm-support branch, not main
• Build validation on ROCm hardware still needed
• 69 tests are skipped on ROCm via rocm_skip.py (FFT, LAPACK, complex types, Metal-specific tests)