[lld-macho] Ensure reads from nlist_64 structs are aligned when necessary

My test refactoring in D80217 seems to have caused yaml2obj to emit
unaligned nlist_64 structs, causing ASAN'd lld to be unhappy. I don't
think this is an issue with yaml2obj though -- llvm-mc also seems to
emit unaligned nlist_64s. This diff makes lld able to safely do aligned
reads under ASAN builds while hopefully creating no overhead for regular
builds on architectures that support unaligned reads.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D80414