AArch64: add support for dynamic-loader relocations

LLD needs them, and it's good to be able to print them properly when
our object dumpers encounter them.

Patch by Daniel Stewart.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215352 91177308-0d34-0410-b5e6-96231b3b80d8