Add internal tracking of smp startup status to reliably figure out
what methods are to be used to get gtaskqueue up and running.

Calculating this pointer gives undefined behaviour when (last == -1)
(it is before the buffer).  The pointer is always followed.  Panics
occurred when it points to an unmapped page.  Otherwise, the pointed-to
garbage tends to not have the E1000_TXD_STAT_DD bit set in it, so in the
broken case the loop was usually null and the function just returned, and
this was acidentally correct.