cdefs: Remove func stub.

Redo 17a238a15fbe. Remove the func crutch for gcc 2.95 and earlier.
We don't need it today to build the tree (since gcc < 12 is unlikely to
work). And it's not used in any system header that's part of the
standard interfaces today (so we don't need it for compatibility). And
we have other issues that make gcc < 4.2 unlikely to work today with
system headers.

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