[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

mlir/CMakeLists.txt

@@ -164,6 +164,8 @@ add_subdirectory(tools/mlir-pdll)
 
 set(MLIR_TABLEGEN_EXE "${MLIR_TABLEGEN_EXE}" CACHE INTERNAL "")
 set(MLIR_TABLEGEN_TARGET "${MLIR_TABLEGEN_TARGET}" CACHE INTERNAL "")
+set(MLIR_PDLL_TABLEGEN_EXE "${MLIR_PDLL_TABLEGEN_EXE}" CACHE INTERNAL "")
+set(MLIR_PDLL_TABLEGEN_TARGET "${MLIR_PDLL_TABLEGEN_TARGET}" CACHE INTERNAL "")
 
 add_subdirectory(include/mlir)
 add_subdirectory(lib)