PATCH: provide pass-through value in bfd_elf_bfd_from_remote_memory

[Roland McGrath]

> This patch changes the interface to bfd_elf_bfd_from_remote_memory to
> allow the caller to pass an opaque data value through to the
> target_read_memory function.  

Fine by me.  If you're going to change the signature, I would change int to
bfd_size_type as well.  I chose the old signature only because it matches
gdb's target_read_memory exactly and that's what I'm using.  It's no
problem to use a wrapper for the calls in gdb.