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

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> it breaks several bots like this https://lab.llvm.org/buildbot/#/builders/164/builds/3908

To clarify for posteriority here as well - it seems like https://github.com/llvm/llvm-project/pull/108357 was the one that broke a buildbot, not this one.

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