[MLton] a better c-types.h?

[Stephen Weeks]

Cross compilation doesn't work with the new runtime includes because
the (automatically generated) c-types.h is platform dependent.  It
used to be that the same includes could be included on any platform,
and automatically handled platform dependencies by C conditional
compilation.

It seems to me that c-types.h could be platform dependent, if instead
of automatically generating a typedef like

  typedef /* dev_t */ Word64_t C_Dev_t;

we simply generated

  typedef dev_t C_Dev_t;

Are there any problems with this?  It seems like a clear win to me.