[llvm-readobj][offload] Fix llvm-readobj --all on MachO (#175912)
Currently running llvm-readobj --all on any MachO object asserts,
because it implies --offload, and the --offload extraction includes an
assert on the object format. Instead, we should be silently ignoring.
This regressed in https://github.com/llvm/llvm-project/pull/143342.