examples/fake-signing: Make runnable on Linux

[MarijnS95]

The main reason for fake signing is making it run on Linux - and not depend on dxil.dll on Windows - so it's great if this example runs on Linux as well to see it all working even if that requires a few nasty cfg blocks. Note that this is done with cfg!() and regular if blocks instead of attributes, such that this code lints and compiles on both Windows and Linux even if the validate_dxil() codepath is never entered on the latter (and that way includes, let mut all_matches etc don't need to be #[cfg(windows)]'ed out either).

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Creating as draft because:

• This violates the validate name, but I think it's worth it being able to run this on Linux;
• Need to figure out why these shaders cause a DefaultIncludeHandler::load_source() for the root directory of this crate (it should only use fules): this doesn't happen when compiling shaders to spir-v;
  For now I can just replace the unwrap with ok()? so that we return -2_147_024_894 // ERROR_FILE_NOT_FOUND / 0x80070002 to DXC; the handler is allowed to be fallible Documented and solved in utils: Propagate IncludeHandler file loading failures to DXC instead of a panic #27;
• (I should drop the "revert virtual dtors" commit - have DXC built with [Linux] WinAdapter: Remove virtual dtors from IUnknown to fix vtable ABI microsoft/DirectXShaderCompiler#3793 which is stuck in ignore-hell).