MFC r256691, r256748: casuword fixes
MFC r256691, r256748: casuword fixes

  Use unsigned compare against KERNBASE addr.
  Use atomic ops on armv6.

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase