message: Change r1 in four_arg_syscall to a u32.
  I mistakenly kept it as a `usize` in my previous commit because memop passes pointers using r1, but four_arg_syscall is not used to call memop. Therefore the second argument can always be a u32.
diff: @@ -116,7 +116,7 @@ pub trait RawSyscalls {
  /// depending on the system call class.
  unsafe fn four_arg_syscall(
  r0: u32,
  - r1: usize,
  + r1: u32,
  r2: usize,
  r3: usize,
  class: u8,
  