[MLton] x86_64 port status

[Stephen Weeks]

Regardign PRIxPTR, perhaps Matthew was suggesting something like

  #ifndef PRIxPTR
  #define PRIxPTR "x"
  #endif

rather than

  #if (defined (__sun__))
  #define PRIxPTR "x"
  #endif

in which case it would make sense to be in cenv.h (assuming "x" is
right for any platform).