Add macros which allow one to define SDT probes with six or seven arguments;
they are needed when porting some of the Solaris providers (ip, iscsi, and
tcp in particular).

dtrace_probe() only takes five arguments from the probe site, so we need to
add the appropriate cast to allow for more than five arguments. The extra
arguments are later copied out of dtrace_probe()'s stack frame by
dtrace_getarg() (or the provider-specific getarg method) as needed.

MFC after: 1 week