Enable KPACK_SPLIT_ARTIFACTS flag

[marbre]

Flip the kpack artifact splitting flag to ON via BRANCH_FLAGS.cmake so that CI builds on this branch produce split (generic + arch-specific) artifacts. The kpack runtime in CLR is already enabled unconditionally; this enables the build-side splitting that produces the archives CLR will load.

Changes:

• Add BRANCH_FLAGS.cmake with therock_override_flag_default(KPACK_SPLIT_ARTIFACTS ON)

[ScottTodd]

Triaged some of the workflow failures:

• non-multi-arch CI builds like https://github.com/ROCm/TheRock/actions/runs/24539771348/job/71742959056?pr=4628 succeeded but packaging tests (ctest in the build dir) have preexisting failures, see [Issue] CI Nightly packaging: RDC shared library load + SONAME validation failing (librdc_client.so) #4640
• non-multi-arch CI python builds on Windows failed: https://github.com/ROCm/TheRock/actions/runs/24539771348/job/71757264636?pr=4628

  [791](https://github.com/ROCm/TheRock/actions/runs/24539771348/job/71757264636?pr=4628#step:6:3792)
  ::: Building python package rocm: 'C:\home\runner\_work\_tool\Python\3.12.10\x64\python.exe' 'C:\home\runner\_work\TheRock\TheRock\packages\rocm\setup.py' sdist -v --dist-dir 'C:\home\runner\_work\TheRock\TheRock\packages\dist'
  Loaded rocm dist_info: version=7.13.0.dev0+c93a77af00cb3d7b59c9a8acda0b1bd491f0f7fb, suffix_nonce='', default_target_family='DEFAULT', available_target_families=[], packages={'meta': rocm, 'core': rocm-sdk-core, 'libraries': rocm-sdk-libraries, 'device': rocm-sdk-device-{target_family}, 'devel': rocm-sdk-devel, 'profiler': rocm-profiler}
  Traceback (most recent call last):
    File "C:\home\runner\_work\TheRock\TheRock\packages\rocm\setup.py", line 51, in <module>
      TARGET_FAMILY = dist_info.determine_target_family()
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "C:\home\runner\_work\TheRock\TheRock\packages\rocm\src\rocm_sdk\_dist_info.py", line 180, in determine_target_family
      raise ValueError(
  ValueError: Requested ROCM_SDK_TARGET_FAMILY=DEFAULT is not available in the distribution (available: )
  

• multi-arch CI Linux builds succeeded, then failed to build pytorch: https://github.com/ROCm/TheRock/actions/runs/24539771367/job/71756383242?pr=4628, see Re-admit hipSOLVER fortran shim in kpack-split devel wheel #4630
• multi-arch CI Windows builds succeeded but have many gfx110X test failures, preexisting? https://github.com/ROCm/TheRock/actions/runs/24539771367/job/71773374223?pr=4628

[ScottTodd]

Closing in favor of #4652