This patch attempts to add __builtin_operator_new/delete. So far they
are not optimized, which will need to be done by extra flag of BUILT_IN_
code.  also the decl.cc code can be refactored to be less of cut&paste
and I guess has_builtin hack to return proper value needs to be moved
to C++ FE.

However the immediate problem I run into is that libstdc++ testuiste
fails due to lack of std::nothrow overrides.  I wonder how to get that
working?