restructure the directory layout

librs/CMakeLists.txt

librs/src/arch/mod.rs

librs/src/arch/x86_64/kernel/CMakeLists.txt

librs/src/arch/x86_64/kernel/acpi.rs

librs/src/arch/x86_64/kernel/apic.rs

librs/src/arch/x86_64/kernel/boot.asm

librs/src/arch/x86_64/kernel/entry.asm

librs/src/arch/x86_64/kernel/gdt.rs

librs/src/arch/x86_64/kernel/idt.rs

librs/src/arch/x86_64/kernel/irq.rs

librs/src/arch/x86_64/kernel/mod.rs

librs/src/arch/x86_64/kernel/pci.rs

librs/src/arch/x86_64/kernel/percore.rs

librs/src/arch/x86_64/kernel/pic.rs

librs/src/arch/x86_64/kernel/pit.rs

librs/src/arch/x86_64/kernel/processor.rs

librs/src/arch/x86_64/kernel/scheduler.rs

librs/src/arch/x86_64/kernel/serial.rs

librs/src/arch/x86_64/kernel/sighandler.asm

librs/src/arch/x86_64/kernel/switch.asm