betterC and core.stdc modules; betterC and varargs routines

[Richard (Rikki) Andrew Cattermole]

Looks like for dmd it defines functions with bodies.

LDC and GDC use builtins instead.

https://github.com/dlang/dmd/blob/f6f520aedeec5f823b9d5dae431d1760a82e8639/druntime/src/core/stdc/stdarg.d#L319

Add ``-i=core.stdc.stdarg`` and it should compile it in.