# Creates fat libs of all .a files in `ARCH1_DIR` (1st param), combining .a with same name in `ARCH2_DIR` (2nd param), saving fat lib in `OUTPUT_DIR` (3rd param).
# Creates fat libs of all .a files in `ARCH1_DIR` (1st param), combining .a with same name in `ARCH2_DIR` (2nd param), saving fat lib in `OUTPUT_DIR` (3rd param).