[llvm-objdump][Offload] Use common offload extraction method

A previous patch introduced a common function used to extract offloading
binaries from an image. Therefore we no longer need to duplicate the
functionality in the `llvm-objdump` implementation. Functionally, this
removes the old warning behaviour when given malformed input. This has
been changed to a hard error, which is effectively the same.

This required a slight tweak in the linker wrapper to filter out the
user passing shared objects directly.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D136796