pmb.build: make sure cross compiler config is appropriate for each given package

Here's the problem: Imagine a queue with 2 packages in it, the 1st
package uses crossdirect and the second uses cross-native. When building
the 1st package, pmb will configure the native chroot for crossdirect
as expected. When it gets to the 2nd package, the chroot env/config
might not be appropriate for actually doing a native cross compile.

This re-inits the cross compiler stuff if the cross compile method
changes while processing the queue.

Another approach that might solve this problem is to not re-use chroots
when building packages... I didn't think this was a good way to go
because it would greatly increase runtime (having to recreate chroots
multiple times)