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.