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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

FWIW, I'd like to backport this to 19.x if possible. But I'll wait for a little while to see whether this turns up any issues, before filing the backport request.

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