[libunwind] [runtimes] Remove explicit -isysroot from the testing configurations on macOS (PR #66265)

[Petr Hosek via cfe-commits]

petrhosek wrote:

This is going to break our builds because we always use the just built Clang (built using the bootstrap build) even on Darwin. I'm fine doing this change for `apple-*.cfg.in` configurations to better match Apple configuration, but for `llvm-*.cfg.in` I'd like to preserve the existing configuration and ideally also land https://reviews.llvm.org/D151056.

https://github.com/llvm/llvm-project/pull/66265