core: move vgafontbuf out of the zero segment

core: move vgafontbuf out of the zero segment

vgafontbuf is one of the largest (bss) structures in the zero segment,
together with the trackbuf (which we realistically can't move.)
Create a new "auxilliary segment" for fixed-sized data which needs to
be in the low megabyte, but not necessarily in the zero segment.

This pushes up the low memory usage, but only by 8K.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>