New RTCD prototype implementation

This is a proof of concept RTCD implementation to replace the current
system of nested includes, prototypes, INVOKE macros, etc. Currently
only the decoder specific functions are implemented in the new system,
and only for x86.

Overview:
  RTCD "functions" are implemented as either a global function pointer
  or a macro (when only one elligable specialization available).
  Functions which have RTCD specializations are listed using a simple
  DSL identifying the function's base name, its prototype, and the
  architecture extensions that specializations are available for.

Advantages over the old system:
  - No INVOKE macros. A call to an RTCD function looks like an ordinary
    function call.
  - No need to pass vtables around.
  - If there is only one elligable function to call, the function is
    called directly, rather than indirecting through a function pointer.
  - Supports the notion of "required" extensions, so in combination with
    the above, on x86_64 if the best function available is sse2 or lower
    it will be called directly, since all x86_64 platforms implement
    sse2.
  - Elides all references to functions which will never be called, which
    could reduce binary size. For example if sse2 is required and there
    are both mmx and sse2 implementations of a certain function, the
    code will have no link time references to the mmx code.
  - Significantly easier to add a new function, just one file to edit.

Disadvantages:
  - Requires global writable data (though this is not a new requirement)
  - 1 new generated source file.

TODO:
  - configure time --disable-* extensions are not yet passed through to
    vpx_rtcd.h generation.
  - configure should support specifying required extensions.

Change-Id: Iae6edab65315f79c168485c96872641c5aa09d55
23 files changed