Reject zero-sized symbols when creating copy relocations.

Copy relocations are relocations to copy data from DSOs to
executable's .bss segment at runtime. It doesn't make sense to
create such relocations for zero-sized symbols.

GNU linkers don't agree with each other. ld rejects such
relocation/symbol pair. gold don't reject that but do not create
copy relocations as well.  I took the former approach because
I don't think the latter is what user wants.

llvm-svn: 270525