[yaml2obj] Support AArch64 relocations.

Patch by Daniel Stewart <stewartd@codeaurora.org>!
Phabricator Revision: http://reviews.llvm.org/D6192

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221639 91177308-0d34-0410-b5e6-96231b3b80d8