[MLton] Problem with MinGW

[Matthew Fluet]

>> At any rate, not only does the c-codegen compile, it succeeds at
>> opening files. Indeed, using the C codegen, MinGW is working
>> perfectly. The moment one uses the native codegen, the problem
>> reappears.
>
> It's a long shot, but I wonder if the "reserveEsp" hack in the native
> codegen is related (see x86-codegen.fun).  There is a test for
> targetOS = Cygwin but not targetOS = MinGW.

I thought the "reserveEsp" hack for Cygwin was for signal handling, since 
Cygwin doesn't provide sigaltstack.  But, it's another good guess where 
the native and C codegens differ.