Add gccxml_builtins.h for GCC 4.9 starting with 4.8 version
Override GCC 4.9 <bits/c++config.h> to disable __int128
Add GCC 4.9 original headers
Update GCC 4.9 headers to work with gccxml's GCC 4.2 parser

Port changes from commit 567213ac (Update GCC 4.8 headers...,
2013-05-02) to GCC 4.9 headers:

* In (|e|x)mmintrin.h remove the function bodies that use builtins not
  provided by the GCC 4.2 parser.  The declarations are sufficient for
  our needs anyway.

* In <complex> remove assignment operator implementations that use
  conversions the older GCC parser does not recognize.

* In <iomanip> remove use of initializer lists.

* In <bits/stl_*> remove use of decltype.

* In <ext/atomicity.h> replace use of atomic builtins with older GCC
  names.

Inspired-by: Mattias Ellert <mattias.ellert@fysast.uu.se>