Macro syscall_table

macro_rules! syscall_table {
    ( $( $name:ident = $num:expr => $func:expr ),* $(,)? ) => { ... };
}

Define syscall table and syscall handler for xv6-riscv64

Example

syscall_table! {
   Invalid = 0 => |_:&mut Xv6Riscv64Abi, _: &mut Trapframe| {
      0
  },
  SomeSyscall = 1 => sys_somecall,
}