Simplify build.sh command line arguments for Linux/arm cross build (Part 1) (#20836)

Simplify build.sh command line arguments for Linux/arm cross build (Part 1) (#20836)

* Don't use crosscomponent|-crosscomponent command line args in build.sh
* Don't use CROSSCOMPONENT environment variable in build.sh src/pal/tools/gen-buildsys-clang.sh
* Remove mentioning -crosscomponent from usage() in build-test.sh
* Don't use -crosscomponent in buildpipeline
* Don't use -crosscomponent in Groovy files
* Remove mentioning crosscomponent in Documentation/building/linux-instructions.md
* Stop building Hostx86/armel crossgen in build.sh
* Disable CrossGen-ing of System.Private.CoreLib on Linux/armel