Extend argument of createIOThread to word size
Function createIOThread expects its second argument to be of size word.
The natural size of the second parameter is 32bits. Thus for some 64bit
architectures, where a write of the lower half of a register does not
clear the upper half, the value must be zero extended.
(cherry picked from commit d0924b153b093a925c9e721f2540f3dfd6c278ae)