Update llvm-objdump’s Mach-O symbolizer code to print the name of symbol stubs.

So in fully linked images when a call is made through a stub it now gets a
comment like the following in the disassembly:

    callq	0x100000f6c             ## symbol stub for: _printf

indicating the call is to a symbol stub and which symbol it is for.  This is
done for branch reference types and seeing if the branch target is in a stub
section and if so using the indirect symbol table entry for that stub and
using that symbol table entries symbol name.