Object: pad out BSD archive members to 8-bytes

ld64 requires its archive members to be 8-byte aligned for 64-bit
content and 4-byte aligned for 32-bit content.  Opt for the larger
alignment requirement.  This ensures that ld64 can consume archives
generated by llvm-ar.

Thanks to Kevin Enderby for the hint about the ld64/cctools behaviours!

Resolves PR28361!

llvm-svn: 294615