gh-91048: Prevent optimizing away the asyncio debug offsets structure on Windows...

gh-91048: Prevent optimizing away the asyncio debug offsets structure on Windows (#132963)

To avoid having the debug sections being optimised away by the compiler
we use  __attribute__((used)) on gcc and clang but in Windows this is
not supported by the Microsoft compiler and there is no equivalent flag.
Unfortunately Windows offers almost no alternative other than exporting
the symbol in the dynamic table or using it somehow.