Move some of signal config to redox-rt.

redox-rt/src/arch/aarch64.rs

redox-rt/src/arch/x86.rs

redox-rt/src/arch/x86_64.rs

redox-rt/src/lib.rs

redox-rt/src/proc.rs

redox-rt/src/signal.rs

src/macros.rs

src/platform/redox/clone.rs

src/platform/redox/signal.rs

src/start.rs