Use x86 crossbuild for cross-architecture component on x64-linux (dotnet/coreclr...

Use x86 crossbuild for cross-architecture component on x64-linux (dotnet/coreclr#8722)

* Use x86 crossbuild for cross-architecture component on x64-linux

Fix build.sh and build system generator to use x86 cross-build system on x64-linux
- x86-host/ARM32-target cross-architecture component build on x64 linux
- use x86 root file system and cross buildtool

* Modify build.sh

- define new function: build_cross_arch_component
- fix typo

* Additional fix: build.sh

add usage description
- environment variable: CAC_ROOTFS_DIR
- argument: crosscomponent

Commit migrated from https://github.com/dotnet/coreclr/commit/f66ba4a14a5e729778d66ea6cc1409f0e8b4a5b0