[c++] builtin functions and namespaces

[Benjamin Kosnik]

FYI I've now enabled -fno-builtin for libstdc++-v3, until this gets sorted
out. Disabling namespaces is now out of the question.

> To achieve this, I believe cp/decl.c:builtin_function should put
> *some* declarations into std::. Specifically, the __builtin_*
> functions probably need to be globally visible, whereas the functions
> mandated by the C++ standard's standard C library should be in std::.
> To achieve this, additional classes (along with BUILT_IN_NORMAL)
> might be needed, or builtin_function could do magic with looking at
> the name of the function.

The shadow header work will take care of making sure the C library
functions are in std:: ... the patches from Steven yeterday make this much
closer to usable. I suspect that's a better way of going about this than
the additional classes thing.

The builtins, however, have completely stumped me. I believe libstdc++-v3
will actually need compiler support for these. I see no reason builtins
should work in a different way than extern "C" functions, as far as
namespaces go.

-benjamin