Source builds need workarounds for sdist, CUDA arches, non-x86 CPU dispatch, and setup.py hardcoding

Building CTranslate2 from source currently needs multiple downstream workarounds. Core upstream issues:

1. No PyPI sdist (highest impact)
   CTranslate2 publishes wheels but not source distributions, which forces custom source-resolution logic.
   Ask: Publish sdists to PyPI.

2. Deprecated FindCUDA path breaks Blackwell arch handling (related: #1933)
   Current FindCUDA/select_compute_arch.cmake behavior drops newer multi-digit CUDA arch majors, requiring manual -gencode fixes.
   Ask: Migrate to FindCUDAToolkit + CMAKE_CUDA_ARCHITECTURES.

3. ENABLE_CPU_DISPATCH breaks on unsupported non-x86/non-arm64 arches
   CPU_ISA_DISPATCH is undefined on targets like ppc64le/s390x.
   Ask: Add support or gracefully disable/fallback on unsupported arches.

4. setup.py hardcodes absolute rpaths (related context: #1704)
   Absolute paths reduce wheel relocatability.
   Ask: Use $ORIGIN-relative rpaths by default.

5. setup.py hardcodes GCC-specific flags (related: #1704)
   Flags like -fvisibility=hidden and -fno-openmp-implicit-rpath can fail on clang/amdclang.
   Ask: Gate flags by compiler/toolchain detection.

These fixes would significantly improve portability and reduce downstream patching.

Review the packaging setup.py and the deprecated FindCUDA/select_compute_arch.cmake path first, then locate ENABLE_CPU_DISPATCH and CPU_ISA_DISPATCH handling for unsupported architectures. Check how source distributions and wheels are configured. Done means sdists publish successfully, CUDA architectures work, unsupported CPU targets fall back cleanly, and rpaths and compiler flags are portable.