ACPICA: OSL: Add configurability for generic external functions.

OSPMs like Linux trend to include all header files but leave empty inline
stub functions for a feature that is not configured during build.
This patch adds wrappers mechanism to be used around ACPICA external
interfaces to facilitate OSPM with such configurability.

This patch doesn't include code for Linux to use this new mechanism, thus
no functional change.  Lv Zheng.
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>