patch for w32 callbacks w/DEP enabled

[Timothy Wall]

When Data Execution Protection (DEP) is enabled, w32 callback memory  
must be allocated via VirtualAlloc, with the execution bit set.   
Otherwise access violation exceptions are thrown when the callback  
trampoline instructions are executed.  I switch some local code over  
to ffi_closure_alloc thinking this was working (since the appropriate  
code is in dlmalloc already).  Turns out none of it is being used, so  
I wound up with a bunch of crashing tests.

This patch enables the right bits when building for windows.

I notice that the mmap (vs vanilla malloc) stuff is otherwise only  
enabled for linux; it should probably be part of the configury to  
detect whether the mmap stuff is required or might be required on a  
particular platform.

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