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

.github/workflows/build-cpu.yml

@@ -119,6 +119,7 @@ jobs:
           ./bin/llama-completion -m stories260K.gguf -p "One day, Lily met a Shoggoth" -n 500 -c 256
 
   windows:
+    name: windows / ${{ matrix.build }}
     runs-on: windows-2025
 
     env:
@@ -130,13 +131,13 @@ jobs:
         include:
           - build: 'x64-cpu-static'
             arch: 'x64'
-            defines: '-G "Ninja Multi-Config" -D CMAKE_TOOLCHAIN_FILE=cmake/x64-windows-llvm.cmake -DGGML_NATIVE=OFF -DLLAMA_BUILD_SERVER=ON -DGGML_RPC=ON -DBUILD_SHARED_LIBS=OFF'
+            defines: '-G "Ninja Multi-Config" -D CMAKE_TOOLCHAIN_FILE=cmake/x64-windows-llvm.cmake -DGGML_NATIVE=OFF -DGGML_OPENMP_FETCH=ON -DLLAMA_BUILD_SERVER=ON -DGGML_RPC=ON -DBUILD_SHARED_LIBS=OFF'
           - build: 'x64-openblas'
             arch: 'x64'
             defines: '-G "Ninja Multi-Config" -D CMAKE_TOOLCHAIN_FILE=cmake/x64-windows-llvm.cmake -DGGML_NATIVE=OFF -DLLAMA_BUILD_SERVER=ON -DGGML_RPC=ON -DGGML_BACKEND_DL=ON -DGGML_CPU_ALL_VARIANTS=ON -DGGML_OPENMP=OFF -DGGML_BLAS=ON -DGGML_BLAS_VENDOR=OpenBLAS -DBLAS_INCLUDE_DIRS="$env:RUNNER_TEMP/openblas/include" -DBLAS_LIBRARIES="$env:RUNNER_TEMP/openblas/lib/openblas.lib"'
           - build: 'arm64'
             arch: 'arm64'
-            defines: '-G "Ninja Multi-Config" -D CMAKE_TOOLCHAIN_FILE=cmake/arm64-windows-llvm.cmake -DGGML_NATIVE=OFF -DLLAMA_BUILD_SERVER=ON'
+            defines: '-G "Ninja Multi-Config" -D CMAKE_TOOLCHAIN_FILE=cmake/arm64-windows-llvm.cmake -DGGML_NATIVE=OFF -DGGML_OPENMP_FETCH=ON -DLLAMA_BUILD_SERVER=ON'
 
     steps:
       - name: Clone