disas: Implement fallback to dump object code as hex

The OBJD-[HT] tags will be used by a script to run the hex blob
through objdump --disassemble.

Signed-off-by: Richard Henderson <rth@twiddle.net>
disas: Add disas-objdump.pl

The script massages the output produced for architectures that are
not supported internally by qemu though an external objdump program
for disassembly.

Signed-off-by: Richard Henderson <rth@twiddle.net>