Add support for 64 bit mode

[coditva]

Summary:

• Setup Grub as the bootloader
• Separate kernel into loader and 64-bit kernel.
• Setup transition from 32 to 64 bit mode in loader.
• Add kernel as a module in Grub and use loader to find the kernel module and jump to it.

refs:

1. https://wiki.osdev.org/X86-64
2. https://wiki.osdev.org/Creating_a_64-bit_kernel
3. https://wiki.osdev.org/Creating_a_64-bit_kernel_using_a_separate_loader