CI: Use LLVM's OpenMP over MSVC_DEBUG_non_redist on Windows (#26678)

* CI: Use LLVM's OpenMP over MSFT_DEBUG_non_redist on Windows

Currently, we ship the non-redist debug version of microsoft's libomp.
This PR changes this to official LLVM's release, also packaging
the license as needed.

* Remove LLVM SHA from job name to increase legibility

* Add temp validations to CI

* Revert "Add temp validations to CI"

This reverts commit eef97c88b5bac280803ebb3c3b7bb09f89b0fd88.

* Build OpenMP in CI

* Make OpenMP fetch self-contained in cmake and cache in CI

* Robustify Licens-packaging

1. Ship OpenMP license, not LLVM's.
2. Invalidate cache also on checksum of the license

* Remove stale reference in docs/build.md

* No longer package base license in release

This was scope-creep

* Add explanatory comment to OpenMP license

* Remove arm64 smoke

Forgot this during conflict resolution during rebase of
c54c0e9cf6030a5a54ce8bdd81b3e146d9787d42

* Remove GGML_OPENMP_FETCH_CACHE_DIR as requested by @CISC

* whitespace changes

ggml/src/ggml-cpu/CMakeLists.txt

@@ -74,7 +74,7 @@ function(ggml_add_cpu_backend_variant_impl tag_name)
 
     if (GGML_OPENMP_ENABLED)
         target_compile_definitions(${GGML_CPU_NAME} PRIVATE GGML_USE_OPENMP)
-        target_link_libraries(${GGML_CPU_NAME} PRIVATE OpenMP::OpenMP_C OpenMP::OpenMP_CXX)
+        target_link_libraries(${GGML_CPU_NAME} PRIVATE ${GGML_OPENMP_TARGET_C} ${GGML_OPENMP_TARGET_CXX})
     endif()
 
     if (GGML_LLAMAFILE)