[llvm-objdump] Complete -chained_fixups support

This commit adds definitions for the `dyld_chained_import*` structs.
The imports array is now printed with `llvm-otool -chained_fixups`. This
completes this option's implementation.

A slight difference from cctools otool is that we don't yet dump the
raw bytes of the imports entries.

When Apple's effort to upstream their chained fixups code continues,
we'll replace this code with the then-upstreamed code. But we need
something in the meantime for testing ld64.lld's chained fixups code.

Differential Revision: https://reviews.llvm.org/D131982