Attempt to fix Symbios driver on amd64. The private memory allocation
function provided by the driver limits allocations to the page size,
i.e. 4KB on i385 and 8KB on typical 64 bit processors. Since amd64
has 64 bit pointers, but only 4KB pages, an array of pointers that
just fits into one page on all the other processors, does require
2 pages on amd64.

In order to make this driver useful on amd64, the allocation unit
has been increased to 2 pages on amd64 and contigmalloc() is used
instead of malloc(). All other processor types are unaffected by
this change. This modification has only been compile-tested on
amd64, yet, but should just work (FLW).