Add signal support - exit on signal

.gitmodules

Makefile

arch/x86_64/src/context.rs

arch/x86_64/src/interrupt/exception.rs

kernel/context/context.rs

kernel/context/switch.rs

kernel/lib.rs

kernel/syscall/driver.rs

kernel/syscall/mod.rs

kernel/syscall/privilege.rs

kernel/syscall/process.rs

libstd

libstd/Cargo.toml

libstd/alloc_system/Cargo.toml

libstd/alloc_system/src/lib.rs

libstd/compiler_builtins/Cargo.toml

libstd/compiler_builtins/src/lib.rs

libstd/libc/Cargo.toml

libstd/libc/build.rs

libstd/libc/src/funcs.rs