llvm-objdump: Print x64 unwind info in executable.

The original code does not work correctly on executable files because the
code is written in such a way that only object files are assumed to be given
to llvm-objdump.

Contents of RuntimeFunction are different between executables and objects. In
executables, fields in RuntimeFunction have actual addresses to unwind info
structures. On the other hand, in object files, the fields have zero value,
but instead there are relocations pointing to the fields, so that Linker will
fill them at link-time.

So, when we are reading an object file, we need to use relocation info to
find the location of unwind info. When executable, we should just look at the
values in RuntimeFunction.