remove idt & idt_reg

[purplewall1206]

"idt.h" included by idt.c and kernel.c, which will cause link failed. so move the definition of idt to idt.c to avoid the failure below.

ld: cpu/idt.o:(.bss+0x0): multiple definition of idt'; kernel/kernel.o:(.bss+0x0): first defined here ld: cpu/idt.o:(.bss+0x800): multiple definition of idt_reg'; kernel/kernel.o:(.bss+0x800): first defined here ld: cpu/isr.o:(.bss+0x0): multiple definition of idt'; kernel/kernel.o:(.bss+0x0): first defined here ld: cpu/isr.o:(.bss+0x800): multiple definition of idt_reg'; kernel/kernel.o:(.bss+0x800): first defined here