ACPICA: Linux: Add stub support for Linux specific variables and functions.

There are global variables and functions not upstreamed to the ACPICA code
base.  Such symbols still can be referenced by external users as they are
listed in the acpixf.h.  This patch uses ACPI_GLOBAL and
ACPI_EXTERNAL_RETURN_STATUS mechanism to add stub support for such symbols.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>