Install i386-elf-gcc using homebrew and use QEMU for simulator

Homebrew often conflicts with MacPorts, and it is a pain for homebrew users to install MacPorts just for this project. Homebrew is better maintained by active open source community 

Instead of installing i386-elf-gcc using macports, I wrote [formulas for gcc cross compiler](https://github.com/jinmel/homebrew-gcc_cross_compilers) to work with homebrew

Simulator was changed to QEMU since bochs is outdated and QEMU has faster performance(except for alarm clock tests)