Add support for extra, optional OS-specific header file.

If OSPMs have something should appear after actypes.h to reference type
definitions, the platform/acxxx.h is not sufficient as it is included by
platform/acenv.h before including actypes.h.

This patch introduce an OSPMs declarable headers to allow OSPMs to handle
such requirement for their own purposes.
This kind of header can also be used by Linux to collect the divergences
that haven't been back ported yet.  Lv Zheng.