Use error-on-mismatch policy for PAuth module flags.
This agrees with Clang, and avoids an error when using LTO with mixed
C/Rust. LLVM considers different behaviour flags to be a mismatch,
even when the flag value itself is the same.
This also makes the flag setting explicit for all uses of
LLVMRustAddModuleFlag.
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I believe that this fixes#92885, but have only reproduced it locally on Linux hosts so cannot confirm that it fixes the issue as reported.
I have not included a test for this because it is covered by an existing test (`src/test/run-make-fulldeps/cross-lang-lto-clang`). It is not without its problems, though:
* The test requires Clang and `--run-clang-based-tests-with=...` to run, and this is not the case on the CI.
* Any test I add would have a similar requirement.
* With this patch applied, the test gets further, but it still fails (for other reasons). I don't think that affects #92885.
13b87d8cc7