If you want to do disassemble of ARM code, you'd better have an ARM tool chain, this is what I got:
http://bb.osmocom.org/trac/wiki/toolchain
After you have this, you can use arm-elf-objdump instead of objdump.
The command I used is
arm-elf-objdump -D -b binary -marm binaryfile.dat
If you look the manpage, you will find "-b" is followed by the file type. Sorry I don't know how to tell -b you want to analyze a .o file. "-marm" will tell the cpu is ARM.
Hope this can help you.
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