[cmake] Fix tablegen exports

This fixes some fallout from D131282. Currently, add_tablegen() will add the tablegen target to LLVM_EXPORTS and associates the install with LLVMExports. For non-standalone builds, this means that you end up with mlir-tblgen and clang-tblgen in LLVMExports.

However, these projects should instead be using MLIR_EXPORTS/MLIRTargets and CLANG_EXPORTS/ClangTargets. To fix this, add an extra EXPORT option and make use of get_target_export_arg() to create the correct export argument.

Reviewed By: ashay-github

Differential Revision: https://reviews.llvm.org/D131565