Honoring std for library functions

[Martin v. Loewis]

I don't know when exactly this broke, but it seems the recent
introduction of build_cp_library_fn and friends broke -fhonor-std.  In
init_exception_processing, the compiler enters std before building the
terminate_node; however, this is ignored later.

I'm not sure what the right fix would be - one option is to set the
DECL_CONTEXT of the new declaration inside build_library_fn_1 if
current_namespace is not global_namespace.

Regards,
Martin