[llvm-objdump] Fix -R display and support ET_EXEC

* Add a newline before `DYNAMIC RELOCATION RECORDS` (see D101796)
* Add the missing `OFFSET TYPE VALUE` line
* Align columns

Note: llvm-readobj/ELFDumper.cpp `loadDynamicTable` has sophisticated PT_DYNAMIC
code which is unavailable in llvm-objdump.

Reviewed By: jhenderson, Higuoxing

Differential Revision: https://reviews.llvm.org/D110595