Added type defs about x64 long mode.

Types include structures of segment descriptors under legacy mode and 64-bit mode. The descriptor under 64-bit is almost reserved and only few bits is considered, making large part of the structure useless.
Ignore file change and reimplemented main.c

The main function of the kernel (that is, the entry point of the 64-bit code) is being reimplementing for a better organization.
Now the main function is empty.