Add foreign import/export implementations for x86 stdcall convention.

Make parser notice calling conventions on f-i and f-x declarations,
check they are supported on the platform Hugs is compiled on.  Pass
them all the way through the code generator to the interpreter.

Allow f-i/f-x decls to omit the calling convention, in which case
ccall is used.  Remove calling convention from all such decls
in the Prelude so it will work on any platform.