Minor bug fix and cleanup in simulator:
  -  removed opcodes whose simulator support are missing: SC, ADDICx, MULLI
  -  added sign-extension of imm in SUBFIC
  -  removed unused helper function
  -  refactoring code to use SIGN_EXT_IMM16() macro for sign extension
remove MULLI from simulator (not implemented in simulator and not used)
fix simulation-ppc build break