[llvm-objdump] Update offload dumping to use SHT_LLVM_OFFLOADING

In order to be more in-line with ELF semantics, a previous patch added
support for a new ELF section type to indicate if a section contains
offloading data. This allows us to now check using this rather than
checking the section name directly. This patch updates the logic to
check the type now instead.

I chose to make this emit a warning if the input is not an ELF-object
file. I could have made the logic fall-back to the section name, but
this offloading in LLVM is currently not supported on any other targets
so it's probably best to emit a warning until we improve support.

Depends on D129052

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D129053