syscall emulation: fix DPRINTF arg ordering bug

syscall emulation: fix DPRINTF arg ordering bug

When we switched getSyscallArg() from explicit arg indices to
the implicit method, some DPRINTF arguments were left as calls
to getSyscallArg(), even though C/C++ doesn't guarantee
anything about the order of invocation of these calls.  As a
result, the args could be printed out in arbitrary orders.

Interestingly, this bug has been around since 2009:
http://repo.gem5.org/gem5/rev/4842482e1bd1