core: move xfer_buf_seg to segment 3, and add a collision assert

core: move xfer_buf_seg to segment 3, and add a collision assert

Move the xfer_buf_seg to segment 3 (0x30000) for now; this is more
generous than I hope we will use but makes development easier.

Add an assert to the linker script that the .auxseg/.lowmem segments
don't collide with xfer_buf_seg.

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