llvm-readobj: Add support for reading relocations in the Android packed format.

This is in preparation for testing lld's upcoming relocation packing
feature (D39152). I have verified that this implementation correctly
unpacks the relocations from a Chromium DSO built with gold and the
Android relocation packer for ARM32 and ARM64.

Differential Revision: https://reviews.llvm.org/D39272

llvm-svn: 316543