[compiler-rt] [compiler-rt] [test] Fix using toolchains that rely on Clang default configs (PR #113491)

[Sam James via llvm-commits]

https://github.com/thesamesam approved this pull request.

First, thanks, and I'm sorry for the breakage. I should've thought of this.

Second, looks good to me, but I'm also happy to `-U_FORTIFY_SOURCE` and such instead if someone really dislikes this approach.

https://github.com/llvm/llvm-project/pull/113491