[llvm-readobj] Add experimental support for SHT_RELR sections

This change adds experimental support for SHT_RELR sections, proposed
here: https://groups.google.com/forum/#!topic/generic-abi/bX460iggiKg

Definitions for the new ELF section type and dynamic array tags, as well
as the encoding used in the new section are all under discussion and are
subject to change. Use with caution!

Author: rahulchaudhry

Differential Revision: https://reviews.llvm.org/D47919

llvm-svn: 335922