Remove Hostx86/arm CrossGen from building and Simplify build.sh command line argument...

Remove Hostx86/arm CrossGen from building and Simplify build.sh command line arguments for Linux/arm cross build (Part 2) (#21034)

* Stop building and publishing Hostx86/arm crossgen on Linux/arm

* Remove -crosscomponent argument and stop using CAC_ROOTFS_DIR environment variable in build.sh

* Simplify the related logic in build.sh

* Don't need to specify crosscomponent in tests/scripts/run-pmi-diffs.py

* Don't set CAC_ROOTFS_DIR in buildpipeline, Jenkins files and in tests/scripts/run-pmi-diffs.py

* Adjust documentation