[MLton] x86_64 port status

[Matthew Fluet]

> <platform.h>
> 2) identifies the target OS and pulls in a target OS specific header,
>   which (recursively) serves a number of roles:
>    a) attempts to compenstate for a broken C99 environment
>    b) defines a feature set
> 3) defines a bunch of utility typedef and functions
> 5) includes "types.h" and "gc.h", which begins to define many types
>   which are only relevant to the MLton runtime and MLton generated
>   executables.
> 5') includes "basis-ffi.h"

To be fair, the new 'platform.h' is also doing a few other misc. items, 
like giving prototypes for GC_* virtual memory functions; I've put them 
here rather than in a gc/*.h file, since they are functions that need to 
be provided by individual platforms, rather than functions provided by the 
GC runtime.