message: Fix how a pthread_create function argument is constructed.
diff: @@ -37,11 +37,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  let fn_ptr = this.read_scalar(start_routine)?.not_undef()?;
  let instance = this.memory.get_fn(fn_ptr)?.as_instance()?;
  - let func_arg = match *arg {
  - rustc_mir::interpret::Operand::Immediate(immediate) => immediate,
  - _ => unreachable!(),
  - };
  - let func_args = [func_arg];
  + let func_arg = this.read_immediate(arg)?;
  + let func_args = [*func_arg];
  let ret_place =
  this.allocate(this.layout_of(this.tcx.types.usize)?, MiriMemoryKind::Machine.into());
  