[llvm-objdump] Add --reloc alias for -r (PR39407)

This addresses PR39407 (https://bugs.llvm.org/show_bug.cgi?id=39407)
improving compatibility with GNU binutils counterparts.

Reviewed By: kristina

Patch by Higuoxing (Xing).

Differential Revision: https://reviews.llvm.org/D53804

llvm-svn: 345703