Define cTargetArch and start to use it rather than ifdefs

Define cTargetArch and start to use it rather than ifdefs
Using Haskell conditionals means the compiler sees all the code, so
there should be less rot of code specific to uncommon arches. Code
for other platforms should still be optimised away, although if we want
to support targetting other arches then we'll need to compile it
for-real anyway.