Simplify x86_64 percpu and GSBASE calculation

linkers/i686.ld

linkers/x86_64.ld

src/arch/x86/gdt.rs

src/arch/x86/paging/mod.rs

src/arch/x86/start.rs

src/arch/x86_64/consts.rs

src/arch/x86_64/gdt.rs

src/arch/x86_64/idt.rs

src/arch/x86_64/interrupt/handler.rs

src/arch/x86_64/paging/mod.rs

src/arch/x86_64/rmm.rs

src/arch/x86_64/start.rs

src/context/arch/x86.rs

src/context/arch/x86_64.rs

src/context/memory.rs

src/lib.rs

src/scheme/proc.rs

targets/i686-unknown-kernel.json

targets/x86_64-unknown-kernel.json