Allow arch to provide higher precision time

src/arch/aarch64/mod.rs

src/arch/aarch64/time.rs

src/arch/x86/device/hpet.rs

src/arch/x86/device/pit.rs

src/arch/x86/mod.rs

src/arch/x86/time.rs

src/arch/x86_64/device/hpet.rs

src/arch/x86_64/device/pit.rs

src/arch/x86_64/mod.rs

src/arch/x86_64/time.rs

src/time.rs