insane: use HOST_ variables, not TARGET_ to determine the cross system

Almost everywhere those are the same, except when making a cross toolchain
where HOST_ is where it's going to run, and TARGET_ is what it's going to
produce.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>