RSDP. Scan the first 1MB on i386 if the sysctl fails,
o Extend struct ACPIrsdp with the ACPI 2.0 fields which involves
changing a prior reserved field into the ACPI revision,
o Only calculate the RSDP checksum on the first 20 bytes to remain
compatible with ACPI 1.0 tables; we don't check the extended
checksum covering the whole table,
o Use the length field in the RSDP to map the RSDP into the address
space so that we don't have to know about future extensions here.
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