Dont let kernel and standalone both be defined at the same time

_KERNEL and _STANDALONE are different things. They cannot both be true
at the same time. If things that are normally visible only to _KERNEL
are needed for the _STANDALONE environment, you need to also make them
visible to _STANDALONE. Often times, this will be just a subset of the
required things for _KERNEL (eg global variables are but one example).

sys/cdefs.h is included by pretty much everything in both the loader
and the kernel, so is the ideal choke point.