kernel: add CONFIG_ARCH_MAPS_ALL_RAM

Some arches like x86 need all memory mapped so that they can
fetch information placed arbitrarily by firmware, like ACPI
tables.

Ensure that if this is the case, the kernel won't accidentally
clobber it by thinking the relevant virtual memory is unused.
Otherwise this has no effect on page frame management.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>