Add cross compiler as an exercise with some guidance and invoke linker scripts with the new toolchain

[tuhdo]

At the moment, the book uses -m32 and -m64 and -m elf_i386 (for ld), which is not a proper way to create an operating system image for a target machine. However, for the purpose of getting to a bootable image as soon as possible, it is the quickest way. However, we cannot ignore the proper way, so it will be integrated as an exercise by:

• Following the cross compiler tutorial on OSDev wiki
• Then integrate the new toolchain to the existing code base.
• If the readers had done it correctly, the new image should behave the same like before.

After that, the Makefiles must be updated to use the new toolchain for invoking the linker scripts in the book.