[llvm-objcopy] Reorder --dump-section for MachO

Reorder `DumpSection` under `handleArgs` in file `MachOObjcopy.cpp`. The
operation to dump a section is now performed before both add and remove
section operations for MachO file format.

Change for the ELF format at D81097. Together fixes
https://bugs.llvm.org/show_bug.cgi?id=44283

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D81123