Move AsmCodeGen.makeFarBranches to PPC.Instr (#709)

Its implementation is totally specific to PPC.
AsmCodeGen.NcgImpl.ncgMakeFarBranches should take account of info tables (#709)

We have to reduce the maximum number of instructions to jump over depending on the number of info tables in a proc.
Merge commit '3cedbfb49996da2f029b4a84ca39f4d21f309813'

* commit '3cedbfb49996da2f029b4a84ca39f4d21f309813':
  AsmCodeGen.NcgImpl.ncgMakeFarBranches should take account of info tables (#709)
  Move AsmCodeGen.makeFarBranches to PPC.Instr (#709)