add $(CrossCompilePrefix) to hp2ps (#7639)
MachRegs.h requires ghcautoconf.h to be included before it (#7591)

Because, in the case of ARM processors, it needs to know what ARM
variant is being used. This patch fixes that in
includes/CodeGen.Platform.hs, otherwise there is a mismatch between
the compiler and llvm.
add CPP
Try to make ghc-pkg and ghc-cabal use the same flags when bootstrapping.

They share modules in Cabal, and unless they use exactly the same
flags GHC recompiles the modules.