Packaging: Virtual package for libllvm (#27200)

Add an abstraction around libllvm to allow libllvm
providers to be specified for all packages.

This is targeting allowing mesa to build against
llvm-amdgpu or intel-llvm or llvm or any other
custom llvm variant that arises for specific GPU
toolchains

etc/spack/defaults/packages.yaml

@@ -34,6 +34,7 @@ packages:
       java: [openjdk, jdk, ibm-java]
       jpeg: [libjpeg-turbo, libjpeg]
       lapack: [openblas, amdlibflame]
+      libllvm: [llvm, llvm-amdgpu]
       lua-lang: [lua, lua-luajit]
       mariadb-client: [mariadb-c-client, mariadb]
       mkl: [intel-mkl]