MFC: r238190

Implement ia64_physmem_alloc() and use it consistently to get memory
before VM has been initialized.
Now that r251221 has been MFC'ed to stable/9 in r251939, merge the remainder
of r251146 previously partially MFC'ed in r251573:

In bce_get_pg_buf(), take advantage of bus_dmamap_load_mbuf_sg(9).