llvm-readobj: fix MachO relocatoin printing a bit.

There were two issues here:
1. At the very least, scattered relocations cannot use the same code to
   determine the corresponding symbol being referred to. For some reason we
   pretend there is no symbol, even when one actually exists in the symtab, so to
   match this behaviour getRelocationSymbol should simply return symbols_end for
   scattered relocations.
2. Printing "-" when we can't get a symbol (including the scattered case, but
   not exclusively), isn't that helpful. In both cases there *is* interesting
   information in that field, so we should print it. As hex will do.

Small part of rdar://problem/17553104

llvm-svn: 212332