loader: Set preferred kenv for acpi.rsdp on arm64

Several years ago, x86 moved from using hints to communicate this
information to using the simpler acpi.rsdp variables. If one compiles
static hints into the kernel, then these hints are ignored. We can
remove this when we branch FreeBSD 15. Thought about BURN_BRIDGES
here, but it's too messy.

Sponsored by: Netflix
Reviewed by: andrew, jhb
Differential Revision: https://reviews.freebsd.org/D35632