[MLIR][ExecutionEngine] Introduce shared library (#87067)

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.

It is alternatively possible to modify the `MLIRExecutionEngine` target
when `LLVM_BUILD_LLVM_DYLIB` is set, to avoid duplicated libraries.