Extend relocation packing to cover arm64.

Extend relocation packing to cover arm64.

Add new delta encoder and signed LEB128 classes to pack relocations with
addends.  Add associated unit tests.

Add packer functions to handle vectors of ELF::Rela structures
(relocations with addends).  Update unit tests.

Templatize elf_file functions to provide versions that handle both
ELF::Rel and ELF::Rela.

Provide 'golden' 64 bit ARM unpacked and packed test data for
elf_file_unittest.  Add gyp targets, actions, and a helper python
script to easily produce these test data files from source using NDK
toolchains.

Tidy terminology in comments, update --help output, and update the
README.TXT.

BUG=385553

Review URL: https://codereview.chromium.org/410933004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@286503 0039d316-1c4b-4281-b951-d872f2087c98