Syscalls!

Changes so far:

• Added gs_info struct, which saves the user stack and holds a pointer to the kernel stack
• Added syscall::init which enables the syscall/sysret instructions and sets up the STAR, LSTAR and SFMASK registers
• Added syscall_entry which builds a UserFrame (a snapshot of the CPU registers on syscall entry) and calls syscall_dispatch
• Added syscall_dispatch, which currently logs the received syscall number and halts (the system call return path is not implemented YET)
• Added UserFrame which holds a CPU register snapshot which gets taken upon entering the kernel