Support fsgsbase at compile time.

Cargo.lock

Cargo.toml

src/arch/x86_64/gdt.rs

src/arch/x86_64/mod.rs

src/arch/x86_64/start.rs

src/context/arch/x86_64.rs

src/context/context.rs

src/context/signal.rs

src/context/switch.rs

src/syscall/process.rs