Add support for ROCm 6

[jabraham17]

Adds support for running on AMD GPUs with ROCm 6. This comes with the temporary stipulation that ROCm 6 support only works the bundled LLVM 18, due to bugs in the LLVM AMDGPU backend.

This PR also adds 3 sub-variables for CHPL_ROCM_PATH, which supports rocm installs with separated depenencies (like spack installs). The new variables are CHPL_ROCM_BITCODE_PATH (for llvm-amdgpu), CHPL_ROCM_INCLUDE_PATH (for hip) and CHPL_ROCM_RUNTIME_PATH (for hsa-rocr-dev)

Testing

• [ ] validate that ROCm 5.5 still works
• [x] start_test test/gpu/native with ROCm 6.0 with COMM=none
• [ ] start_test test/gpu/native with ROCm 6.1 with COMM=none
• [ ] start_test test/gpu/native with ROCm 6.2 with COMM=none
• [ ] start_test test/gpu/native with ROCm 6.1 with COMM=none and unified memory
• [ ] start_test test/gpu/native with ROCm 6.2 with COMM=ofi
• [ ] checked built docs