loader: fix EFI ACPI detection

lua was previously unable to determine ACPI presence because this
probing was postponed until the final loading and execution of the
kernel.

This patch resolves that by detecting ACPI early (similar to
the order of operations in the legacy i386 loader).

Reviewed by: kevans
Approved by: kp
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D42459

(cherry picked from commit 0b01d45783c3ee5a544c882d1b147e4a60382c41)