Allow linking to LLVM shared library.

[KOLANICH]

Currently when LLVM enabled, symengine links to LLVM static libraries. It is a bug not in symengine, but in LLVM itself (llvm/llvm-project#62846), but it severely affects symengine raising its shared library size from 5.4 MiB stripped to ~57 MiB. So, I guess before LLVM project has it properly implemented, we have to work around it.

Here are examples how various projects work around the issue:

• https://github.com/pocl/pocl/blob/master/cmake/LLVM.cmake
• https://github.com/openai/triton/blob/main/cmake/FindLLVM.cmake
• https://github.com/haoxintu/FastKLEE/blob/main/cmake/find_llvm.cmake