New objcopy option: --alt-machine-code

[Alexandre Oliva]

After we switched the ELF machine code of one of our tool-chains to an
officially-assigned number, one of our customers asked us whether
there was any way to generate binaries that could still be recognized
by old tool-chains, or by their own proprietary tools.  Since AFAIK
there wasn't any easy way to do it, I came up with this patch, that
Nick Clifton already approved, so I'm checking it in.