x86: correctly limit max memory resource address..

CPU and buses can manage up to the limit reported by cpu_maxphyaddr,
so set mem_rman to the value returned by cpu_getmaxphyaddr(). For the
PAE mode, it was missed both when rman_res_t was increased to
uintmax_t, and from the PAE merge commit.

When importing smaps or dump_avail chunks into memory rman, do not
blindly ignore resources which ends above the limit, chomp them
instead if start is below the limit. The same change was already done
to i386 add_physmap_entry().

Based on the submission by: bde
MFC after: 2 months