ghc-toolchain: If a user specificed CPP flags don't second-guess them

When we call ghc-toolchain from ./configure we pass only some of hs-cpp flags.

For example on windows we pass -I/path/to/mingw/include, so the HaskellCPP args are taken to be just this include option (omitting -E and any clang specific options the script would have discovered).

This works differently to things such as the linker, where we test to see if the linker supports certain things.

• Either, pass all the HaskellCppArgs to which ./configure discovers to ghc-toolchain (this seems to defeat the point of ghc-toolchain)
• Make ghc-toolchain still discover arguments even if the user explicitly provided options.
• Explicitly set all the flags needed for the windows toolchain when setting it up rather than just one include directory.

cc @alt-romes