Increased VM_PHSYSSEG_MAX on i386

Trying to fix an error "Too many segments in the physical address
map, giving up" on boot that is seen on certain hardware
configurations when booting the i386 GENERIC kernel.

Previously attempted to work around this by setting hw.physmem to 3G,
but that only resulted in a kernel segfault instead of the too many
segments error.

Using VM_PHYSSEG_MAX set to 31, as taken from the AMD64 configuration,
initial testing seems to indicate this is OK for use on i386 as well.