NCG: Split up the native code generator into arch specific modules

NCG: Split up the native code generator into arch specific modules

  - nativeGen/Instruction defines a type class for a generic
    instruction set. Each of the instruction sets we have,
    X86, PPC and SPARC are instances of it.

  - The register alloctors use this type class when they need
    info about a certain register or instruction, such as
    regUsage, mkSpillInstr, mkJumpInstr, patchRegs..

  - nativeGen/Platform defines some data types enumerating
    the architectures and operating systems supported by the
    native code generator.

  - DynFlags now keeps track of the current build platform, and
    the PositionIndependentCode module uses this to decide what
    to do instead of relying of #ifdefs.

  - It's not totally retargetable yet. Some info info about the
    build target is still hardwired, but I've tried to contain
    most of it to a single module, TargetRegs.

  - Moved the SPILL and RELOAD instructions into LiveInstr.

  - Reg and RegClass now have their own modules, and are shared
    across all architectures.