[llvm-objdump] Ensure offloading sections have proper alignment

Summary:
A previous patch added support for dumping offloading sections. The
tests for this feature added dummy input to the required section using
`llvm-objcopy`. This binary format has a required alignment of `8` which
was not being respected by the file copied with llvm-objcopy and would
cause failures on architectures sensitive to alignment problems or with
sanitizers. This patch adds the proper alignemnt and adds an error check
at least for the binary format so it's not completely opaque. This
should be improvbed so users actually get a helpful message.