loader/4th: Move from hint.acpi.0.rsdp to acpi.rsdp
Use acpi.rsdp to test for ACPI. 4th wasn't updated with the old compat
was dropped a while ago.
MFC After: 1 week
Sponsored by: Netflix
(cherry picked from commit 37798b1d5dd12cd5e842b6f99135a2e4af8cf9e0)