Make top level modules for all the C standard library headers.
The __stddef implementation headers need header guards now that they're all modular. stdarg.h and stddef.h will be textual headers in the builtin modules, and so need to be repeatedly included in both the system and builtin module case. Define their header guards for consistency, but ignore them when building with modules.
__stddef_null.h needs to ignore its header guard when modules aren't being used to fulfill its redefinition obligation.
__stddef_nullptr_t.h needs to add a guard for C23 so that _Builtin_stddef can compile in C17 and earlier modes. _Builtin_stddef.nullptr_t can't require C23 because it also needs to be usable from C++.