ARM26 define FIRST_USER_ADDRESS as PAGE_SIZE (beyond the machine vectors when
they are mapped low), and use that definition in place of locally defined
MIN_MAP_ADDR. Previously, ARM26 permitted user mappings at 0 if the machine
vectors were mapped high; but that's inconsistent with ARM, and
FIRST_USER_ADDRESS would then have to be determined at runtime. Let's fix it
at PAGE_SIZE throughout the architecture.