Do not #include external header files when compiling via C

Do not #include external header files when compiling via C

This has several advantages:

 - -fvia-C is consistent with -fasm with respect to FFI declarations:
   both bind to the ABI, not the API.

 - foreign calls can now be inlined freely across module boundaries, since
   a header file is not required when compiling the call.

 - bootstrapping via C will be more reliable, because this difference
   in behavour between the two backends has been removed.

There is one disadvantage:

 - we get no checking by the C compiler that the FFI declaration
   is correct.

So now, the c-includes field in a .cabal file is always ignored by
GHC, as are header files specified in an FFI declaration.  This was
previously the case only for -fasm compilations, now it is also the
case for -fvia-C too.