Tidy up ghc/includes/Constants and related things.

Now all the constants that the compiler needs to know, such as header
size, update frame size, info table size and so on are generated
automatically into a header file, DeriviedConstants.h, by a small C
program in the same way as NativeDefs.h.  The C code in the RTS is
expected to use sizeof() directly (it already does).

Also tidied up the constants in MachDeps.h - all the constants
representing the sizes of various types are named SIZEOF_<foo>, to
match the constants defined in config.h.  PrelStorable.lhs now doesn't
contain any special knowledge about GHC's conventions as regards the
size of certain types, this is all in MachDeps.h.