obj2asm thinks / starts a switch on non-Windows systems

**Transferred from**: https://issues.dlang.org/show_bug.cgi?id=7677

User @WalterBright reported (2012-Mar-09 11:41:05):
> ```shell
> $ obj2asm /bin/ls
> Fatal error: unrecognized flag '/bin/ls'
> ```
> Reported:
>
> http://www.reddit.com/r/programming/comments/qlssq/one_small_step_toward_reducing_programming/c3z7h7v