- copypage() -- a single page-aligned NBPG-byte copy.
- zeropage() -- a single page-aligned NBPG-byte zero.
These functions don't play around with alignment, etc. Their use
causes a measureable performance improvement in pmap_copy_page()
and pmap_copy_page().
A few m68k ports already had copypage() in their locore.s. It has
been moved here so it can be shared.