Emit __VA_ARGS__-using macros into embed.h when on PERL_CORE

Emit __VA_ARGS__-using macros into embed.h when on PERL_CORE

Now we're using C99, we can safely use the __VA_ARGS__ expansion in
these variable-list macros.

Unfortunately we can't just emit them unconditionally, because much
existing CPAN code exists that thinks it can call e.g. `warn()` without
an aTHX_ in scope (because they don't #define PERL_NO_GET_CONTEXT).

Therefore, we have to guard these new macro forms by

  ... || defined(PERL_CORE)

and continue to emit the "..._nocontext()" variants at the end of the
file, as we previously did.

It's not a great solution but it at least means we can use `croak()`,
`warn()`, et.al. within perl core source now.