Implemented sane default for llvm-objdump's relocation Value format

Summary:
"Unknown" for platforms that were not manually added into the switch
did not make sense at all. Now it prints Target + addend for all
elf-machines that were not explicitly mentioned.

Addresses PR21059 and PR25124.

Original author: fedor.sergeev

Reviewers: jyknight, espindola, fedor.sergeev

Reviewed By: jyknight

Subscribers: eraman, dcederman, jfb, dschuff, aheejin, llvm-commits

Differential Revision: https://reviews.llvm.org/D36464

llvm-svn: 333726