FIX rts build failure for powerPC build

FIX rts build failure for powerPC build

The rts was failing with

../compiler/ghc-inplace -H64m -Onot -fasm -optc-O2 -static -I../gmp/gmpbuild -I. -#include HCIncludes.h -dcmm-lint  -hisuf thr_p_hi -hcsuf thr_p_hc -osuf thr_p_o -optc-DTHREADED_RTS -prof -#include posix/Itimer.h  -c PrimOps.cmm -o PrimOps.thr_p_o
ghc-6.7.20070709: panic! (the 'impossible' happened)
  (GHC version 6.7.20070709 for powerpc-apple-darwin):
        iselExpr64(powerpc) %MO_U_Conv_I32_I64(16 / 4 - 2)

There was a special case for x86; so it has been transliterated to the PPC, and the output code looks plausable.