[mlir] Fix calling the native mlir-tblgen tool when cross compiling flang

When the mlir-tblgen tool is set up, the `MLIR_TABLEGEN_EXE` variable
is set, which either points to the mlir-tblgen tool built in the current
cmake build, or points to one built in a nested cmake build (if cross
conpiling, or if building with e.g. `LLVM_OPTIMIZED_TABLEGEN`.

The `MLIR_TABLEGEN_EXE` variable is only set within the scope of the
mlir/CMakeLists.txt file, so it's unavailable in sibling level projects
such as flang.

Set the `MLIR_TABLEGEN_EXE` and the `MLIR_TABLEGEN_TARGET` variables
as global, so that flang can use them properly without guessing.

Differential Revision: https://reviews.llvm.org/D130350