cbprintf: use type instead of name in sizeof

Using `arg + 0` in sizeof causes problems with `void *`, so use the
type name instead, but make sure it's at least `sizeof(int)` because
the variadic expects a minimum of `int` size. This allows deleting
the specialization for `void *`, which the linker wasn't choosing
reliably anyway.

Signed-off-by: Paul Fagerburg <pfagerburg@google.com>