[MLIR][ExecutionEngine] Introduce shared library

This commit introduces a shared library for the MLIR execution engine.
This library is only built when `LLVM_BUILD_LLVM_DYLIB` is set. Having
such a library allows downstream users to depend on the execution engine
without giving up dynamic linkage. This is especially important for CPU
runner style tools, as they link against large parts of MLIR and LLVM.