llvmpipe: make unnamed global have internal linkage

Alex Fan requested to merge alexfanqi/mesa:fix into main

work around orcjit/llvm bug https://github.com/llvm/llvm-project/issues/54813 Being unnamed makes it not useable from other module, therefore changing to internal linkage is safe

This is needed for switching the JIT API used by llvmpipe from outdated MCJIT to ORCJIT that is being worked on in !17801 (closed)

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