insane.bbclass: fix HOST_ variable names

Commit cd25e554 ("insane: use HOST_ variables, not TARGET_ to
determine the cross system") updated the variables themselves, but not
their names. To prevent confusion, match the Python variable name to the
BitBake variable name.

Signed-off-by: Emil Kronborg <emil.kronborg@protonmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit f5bebc96)
Signed-off-by: Steve Sakoman <steve@sakoman.com>