undef _BIG_ENDIAN only if the compiler defines it

to reduce command-line spam
Add -N(ative), or -N(etBSD)

Essentially it sets build variables so that even non-kernel components
built by rumpmake are built like they would be on NetBSD, i.e.
-D__NetBSD__ is there, -Ulinux if necessary, etc.

You probably don't want to use this switch unless you really know what
you're doing.