[mlir] Fix calling the native mlir-pdll-tblgen

This sets the `MLIR_PDLL_TABLEGEN_EXE` and `MLIR_PDLL_TABLEGEN_TARGET`
as cache variables which is necessary for cross-compiling projects that
rely on MLIR and the mlir-pdll-tblgen tool.

The patch is similar to https://reviews.llvm.org/D130350.

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D131596