Fix exported MLIR_TABLEGEN_EXE

LLVM_OPTIMIZED_TABLEGEN results in MLIR_TABLEGEN_EXE pointing to
an absolute path in the build directory.  This doesn't work
when exporting to an install directory.  This patch fixes the exported
information for an install directory to refer to the installed
mlir-tblgen.  (Note that this is probably a debug version if
LLVM_OPTIMIZED_TABLEGEN is set)