x86: memory map BIOS Data Area

Changes to paging code ensured that the NULL virtual page is
never mapped. Since RAM is identity mapped, on a PC-like
system accessing the BIOS Data Area in the first 4K requires
a memory mapping. We need to read this to probe the ACPI RSDP.

Additionally check that the BDA has something in it as well
and not a bunch of zeroes.

It is unclear whether this function is truly safe on UEFI
systems, but that is for another day.

Fixes: #27867

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