[mlir-cpu-runner] Add export_executable_symbols in CMake.

LLJIT needs access to symbols (e.g. llvm_orc_registerEHFrameSectionWrapper)
that will be defined in the executable when LLVM is linked statically.

This change is consistent with how other tools within LLVM use LLJIT. It
is required to make sure that `mlir-cpu-runner --host-supports-jit`
correctly returns `true` on platforms that do support JITting (in my
case that's AArch64 Linux).

See https://github.com/llvm/llvm-project/issues/61712 for more context.

Differential Revision: https://reviews.llvm.org/D146935