Fix the use of lwz and stw w/ non-constant immediate values that may
exceed 16-bit signed immediate
  - Added 3 macroAssembler interfaces to deal w/ :
    LoadSignedImmediate, LoadFromBaseAndOffset, and StoreToBaseAndOffset
  - Added x-form (x-form of load-word)
  - Fixed an encoding bug w/ STWX and STWUX