[llvm-readobj] Make some commonly used short options visibile in -help

For people who use llvm-readelf as a replacement of GNU readelf, they would like to see -d -r ... listed in llvm-readelf -help. It also helps understanding the confusing -s (which is unfortunately different in semantics).

Reviewers: phosek, ruiu, echristo

Reviewed By: ruiu, echristo

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52129

llvm-svn: 342339