[MLton] commit r4732: Generate .symbol_stub sections on x86-darwin

[Matthew Fluet]

The MacOS X assembler guide describes how to generate symbol stubs for
undefined functions that are called in the module.

http://tuvix.apple.com/documentation/DeveloperTools/Reference/Assembler/Assembler.pdf

This is untested, since I don't have access to an Intel Mac, but the
produced assembly appears to match the spec.  I believe that this
should accomodate the _import syntax.

I don't know if anything special needs to be done on x86-darwin to
access a symbol defined in a dynamic library; i.e., the _symbol and
_address syntax.  I'd be interested to know how gcc compiles:

extern int x;
void setX(int y) { x = y; }
int getX() { return x; }
int* getXAddr() { return &x; }


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U   mlton/trunk/mlton/codegen/x86-codegen/x86-allocate-registers.fun
U   mlton/trunk/mlton/codegen/x86-codegen/x86-generate-transfers.fun
U   mlton/trunk/mlton/codegen/x86-codegen/x86-translate.fun
U   mlton/trunk/mlton/codegen/x86-codegen/x86-validate.fun
U   mlton/trunk/mlton/codegen/x86-codegen/x86.fun
U   mlton/trunk/mlton/codegen/x86-codegen/x86.sig