[llvm-objcopy] Add RISC-V support for -B/-O

Reviewers: jorgbrown, espindola, alexshap, jhenderson

Subscribers: emaste, arichardson, fedor.sergeev, jakehehrlich, kito-cheng, shiva0217, MaskRay, rogfer01, rkruppe, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61272

llvm-svn: 359568