Fix a bug in the way a sixth USDT or PID probe argument was fetched.

The DTrace code assumes that the fields of struct reg are ordered in
such a way that r->r_rdi[argno] retrieves the argno'th argument of a
function, for 0 <= argno <= 5. This isn't true on FreeBSD. Changing the
ordering in struct reg will break userland programs which use the struct
definition (debuggers and so on), so just hardcode the argno <->
register mapping in a couple of places.