Properly set the return value for casueword to 0 upon success.

While trying to get multithreading working for CloudABI on aarch64, I
noticed that compare-and-exchange operations in kernelspace would always
fail. It turns out that we don't properly set the return value to 0 when
the compare and exchange succeeds.

Approved by: andrew
Differential Revision: https://reviews.freebsd.org/D3899