[ELF2/AArch64] Read the right amount of bytes.

This was clearly wrong (thanks Rui for spotting), and I honestly would 
like to get this tested so such mistakes won't repeat. Unfortunately, I
wasn't (easily) able to craft a test that exposes the bad behavior.
Ideally, we would like to get tests of this kind for all relocations, but
at the time of writing, this is not true. So, for now just fix this bug
and try to re-evaluate a way to test this in the future.

llvm-svn: 249359