LinuxKPI: acpi: provide union [linuxkpi]_acpi_(object|buffer)

Provide union acpi_(object|buffer) as linuxkpi_ versions and in Linux
native lower case spelling so that driver code compiles unmodified.

Use the linuxkpi_ prefix to avoid name clashes with the native ACPI
implementation which uses CamelCase.

Use the linuxkpi_ names internally and redefine the linuxkpi_ unions
to their native names so they are avail as expected in drivers.

As a transition to not break drm-kmod which has in-place (no #ifdef)
changes from Linux spelling ot FreeBSD spelling, introduce
LINUXKPI_WANT_LINUX_ACPI so we have the Linux spelling as an opt-in.
That way the transition can happen gracefully and once "top-of-trees"
are all sorted we can remove the #ifdefs here again.

Bump __FreeBSD_version to be able to detect this change.

Sponsored by: The FreeBSD Foundation
Reviewed by: wulf (previous version), dumbbell
Differential Revision: https://reviews.freebsd.org/D50863

(cherry picked from commit 2ddfcc013c251f13c8c9b24970b8134ddd49afea)